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I have dealt with this. in 99% of cases, it is either the flow orifice on the mac valve or the limit switch on the bottom of the trans that only "indicates" the gear selection. You need to first verify if the trans actually shifts gears or not. This is pretty easy to verify by trying the spin the spindle by hand. high gear = easy, low gear = harder.
If the trans is not shifting, there are 2 copper lines going to the trans. Pull them both at the mac valves, find the small orifice "pills" between the mac valve and feed line, and try an MDI gear shift. If you get air from the mac, you found your problem. I got so tired of it, I drilled the orifice slightly over sized, problem solved for good. The orifice serves to soften the gear change a bit. Do NOT getting nutty on the drill. .001-.004 oversize will do it.
Mine does seem to clank into gear a bit more now but I doubt anyone else would even notice. The right way to do this should have been another pressure reg with full pilot pressure to the solenoid. faster and softer at the same time.