I'm not too sure how popular this is, but I have seen it personally and recently here on BC. Where cars won't engage into gear without revving the engine excessively. I understand this is beacuse there is a lack of pressure to engage into gear, but what really fails? Is it the main pump? If that were it, wouldn't it just cease engagement all together? If there is sufficient fluid, and everything is all good and well in the transmission... can the modulator'* spring completely fail causing fluid pressure loss, thus causing the car not to engage except at high RPM when the modulator system cannot bypass all the extra fluid and the extra pressure that cannot be bypassed is enough to engage the transmission? It seems to make sense in my mind, but i'm just curious. Obviously this is not just a one time issue, so maybe a major transmission swap or rebuild job could be avoided by a $20 modulator.

Posting under 87-91 I'm assuming thats the era... Could be some of the slides and valves are sticking inside from wear and tear.

Yeah I am talking about the THM-440, however I believe this can apply to any hydraulic transmission?

The thing that makes me stray from there, is that it still engages? If those were sticking.. wouldn't it just take one time to un stick them, then all would be well? I don't know, I'm just kinda typing and thinking at the same time.

could be a cable linkage adjustment problem. There is a procedure for putting it in neutral and setting it.
Theres a 93 Bonne in my neighbors driveway with the same problem.
Won't go into gear. Meant to get over there and try the adjustment. Or at least try putting the shifter slowly between gears and see if it engages.