Load calculation...
I've got a forklift, use many different types all the time... also have a tractor with front-end loader, have a pair of forks that I can put on any one of several tractors, that just use the 3-point (no mast). Have also used skid-steers with forks, and forks-on-bucket...
When I need to load, the first thing I look at, is the weight of the load, and determine it's center-of-gravity, then look at where it's coming FROM, and going TO.
OP's notes suggested somewhere around 4000-5000lbs, palletized, loading onto a semi... I don't recall if he identifed the shape.
but let's say you've got a 60" square pallet that has a 5000lb load weight who's CG is a foot above the pallet top, centered in the 60" pallet...
That means, when you place it ALL the way on the forks, your CG will be 3' past the face of the fork, and 1' above. And with the semi-loading situation, you'll need to lift up about 6', and reach OVER the deck of the trailer... if you want to get to the middle of the trailer, you're looking at reaching in about 3' or so.
With most forklifts, this won't be a difficult situation to sort out... just look at the dataplate, and see if it has load limit specified for a 36" load center... typically, they'll have 24 and 48, so you can get a fair feel for what 36" will yield.
Now, keep in mind that the stability of the forklift is determined by the balance of a triangle shape that goes from the center of each mast-end wheel, to the center of the steering-axle pivot. Let's say the load is 36" out from the face, which is 12" from the edge of the tires, and the tires are 40" in diameter, so the load center, from the point where the tires contact the ground, is 36+12+20=68".
The forklift has a counterweight that is on the far end. Most forklifts I've worked on and with, if the counterweight is removed, the forklift will present MOST of it's weight on the mast-end wheels, but still sit flat. If you tip the mast forward, most will pick up the front wheels and fall forward on the forks. This tells you about how the machine's operating balance occurs... and from a fore-aft perspective, it's all a function of ratios and counterweight. This ALSO tells you that when you tilt the mast (and load) forward, your counterweight reaction is HIGHER... because you have the load center moving farther away from the tire contact point (the triangle), and also, because the weight of the mast is considerable.
Then there's lateral stability... the forklift's support triangle is only-so-wide. In an ideal scenario, the forklift will NEVER be in anything other than a horizontal surface... but I've yet to find an entrance apron to a building that was flat, while at the same time, capable of pushing rain away from the building.... SO... one has to be aware of the load presenting a lateral shift in balance to the machine... again, it's all about where the CG appears with respect to the support triangle. IF you have the load elevated 10' above the machine, that compounds the reaction necessary to maintain stability, and at the same time, concentrates the load on the downhill tire, which, if it's on surface unable to support the pressure, the tire sinks, further increasing load angle, rapidly sending the machine horizontal.
Now consider the load placed on the axles. A forklift's steering axle is loaded heaviest when there's nothing on the forks, sitting still, or worse, with fully loaded forks, extended to full height, with mast tilted all the way back. [I've never done this, and can't think of a reason why I'd ever do it, even if that sort of lift was required...] Now consider the highest load found on the DRIVE(mast) end. When the forks are loaded to the point where the STEER tires are off the ground. What's the load on those wheels? Easy: Weight of the forklift, plus weight of the load. On my Clark IT60, that number comes out around 24,000lbs between the two tires. The IT60 is a pneumatic super-single tire approx 34" diameter, 18" wide, with a backhoe-shaped construction tread... now holding 12,000lbs a tire. Although it's a good, stout tire, and on a large amount of surface, The ground surface underneath that tire is carrying a whole lotta pressure, and of course, the machine's axle is too...
So where's this going?
Compare the load triangle of a forklift, to the load-triangle of a utility tractor with front end loader bucket, with forks. You'll see that the face of the forks, are MUCH farther away from the front axle. The load distance is so far out, that the amount of counterweight required is very high... and all that load shows up on the STEER axle of the tractor.
My dad's little Kubota BX2300(?) is a handy little dude, but when it comes to lifting, it's an absolute wimp, and for really good reason: The boom is LONG, so when you're scooping mulch into a dump-trailer, you can lift up and over the side of the trailer bed... and if you lift too much, you'd be putting a LOTTA load on a nifty, but rather expensive-to-repair 4wd front axle sitting on two rather lame turf tires. My buddy's bigger Kubota (50hp range or so), is for all practical purposes, the same machine in philosophy, only scaled up... and when I pull the bucket up, I know one thing for certain- it will not waste any time in hitting the pressure relief valve limit... and if I try to 'sneak' it up a little more, the machine will NEED the front drive axle engaged in order to move... which is a bad thing, because that means the load now owns you.
Quick-attach systems are cool- nice to be able to pop the bucket off, and slap on forks, or a bale spear, or a hydraulic concrete demolition hammer... and for forks, that means you have the load closer to the machine's axle, so favorable geometry. The downside, is that the additional metal required for the QA, makes both the loader frame AND the implement heavier than if it were purpose-built for the situation. Wouldn't seem like much, but for an average skid-steer, the QA plate on the bucket alone, is frequently close to the comparable wieght of a conventional bucket.
Lifting with skid-steers... the biggest advantage to lifting with a skid-steer, is that if you're at the side of the truck, and the load is crooked, you can pivot (albeit usually not gently) to correct the angle of the load. Depending on the length of forks and boom reach, you may not be able to reach the center of the trailer as well as other options, but you'll be able to see the load from the underside, provided the deck of the trailer isn't staring you in the face. The other advantage to a skid-steer, is that the overall length is short, so more maneuverable in comparison to say... a telehandler or conventional forklift... but they're certainly not gentle, and I have a gentle touch with forklifts...
Another option which nobody mentioned, is to find an articulating loader, remove the bucket, and fit a set of forks. The articulating loader has it's advantages and disadvantages... the most safety-critical being that since it pivots in the middle, the relationship of the load to the counterweight makes for a support triangle who's footprint changes with steering angle. IF you're not lifting anywhere near the machine's capacity, it'll never be a problem, but if you DO approach it's limits, it will git you. Up-side, is that they're pretty sure footed on most conditions, and can make some other tasks incredibly easy... ripping out trees, repairing the driveway, moving snow, extracting stuck forklifts... and when found used, they're frequently found rather cheap... elevated seating position makes for good visibility for loading a truck, but they don't duck under garage doors well. If you get a run at it, though... the door won't stop it from coming inside...
Another possible solution, is to set four large posts in a dedicated spot in the driveway, place beams across the tops, install a rolling gantry, then have the truck back under the gantry to be loaded with overhead hoist. No fuel required...