It would have been a stretch for me to attempt this repair myself so I took it to a shop.

Typical failure for this tranny is losing reverse.

Rebuilt installed for $1650. He put in a new rear seal while he was there. Car shifts great now. Actually it always had shifted perfectly, just was losing reverse.

FWIW, the guy said don't waste money on fancy fluid like Mobil 1. Plain old Dextron will work fine. The most IMPORTANT thing you can do is change fluid regularly like 24k-30k for mechanical trannies and maybe a bit more, like 40m, for electronic ones. Trannies coming in without fluid changes (i.e. lifetime fill ~60m+) are coming in thrashed, just worn out. He suspects that because of the long interval of change, the fluid in there becomes loaded with particles which act as abrasives against any and all internal parts.