1989 ranger extended cab 2.9 reverse slipping

[Donniebbig]

Hi everyone, this is my first Ford Ranger and also my first time here.
I bought the vehicle about a year ago with fairly low miles. 129k. 1989 with the 2.9 and Automatic transmission.
For a few months now I've been noticing a weird downshifting symptom. It stays in the higher gear too long and sort of clunks when it finally grabs the lower gear. It's not terrible yet but noticable. Now I'm noticing that when I try to back up it's slipping if there's any incline at all. If the road is flat it works fine enough.
Any thoughts? Quick fixes? I really don't want to but a new transmission

 

[Spleen]

I’m going to assume you checked the fluid, but that’s the first place to start (in park with it up to temperature). If the fluid is to level and doesn’t look dark and burnt up my next move would be to pull the vacuum line running up from the vacuum modulator on the trans and seeing if there’s fluid in the line, once you locate that line it’ll be all of 10 seconds to yank it off it’s vacuum source, if there’s fluid in it that assures you the vacuum modulator isn’t functioning correctly (known to go bad on the A4LD and being a known crappy transmission people often assume vacuum modulator related issues are worse internal issues). If that checks out Id still consider replacing the modulator as it’s cheap but if that checks out only then would I drop the pan and change the fluid and filter.