125 THM Tranny - Oddball Problem

[TheOneEyedBandit]

Hi, guys. I've got a 1981 Oldsmobile Omega, with the old Turbo-Hydramatic 125 transmission (basically ubiquitous in 1980s GM FWDs). It's doing a very strange thing: there's no reverse (acts like neutral), AND there's no engine braking in lo gear. Acceleration is normal; it pulls forward in all three gears. When I downshift to 2nd, there's engine braking, but when I go to Lo gear, it lets go, like neutral. It'll pull when I give it gas, but when I let up, it just coasts.
I'm not great with automatic transmissions, and I'm not sure what's going on here, but I want to see if it can be fixed without pulling the tranny (I've done that before, and it was no fun), and if the problem might just be something I can actually handle.
Thanks, everyone.