Transmission slipping in reverse

My ’95 Taurus is slipping in reverse. Although I haven’t ‘floored’ the car to check—it doesn’t seem to be slipping in any of the forward gears. I have changed the fluid and filter with no improvement. The fluid was pretty worn, so I’ll drive it a few days and change it again, so I can get more of the old stuff out.
Could this be a sensor—or is the trans starting to fail?
Also, What is under the cover on the rear / lower trans housing? I read that there is a valve or servo under there that sometimes has a bad seal or broken spring. What kind of symptoms occur when something goes wrong in there??
Thanks,
TOM

Have you searched the Taurus Forum? I remember there being a post that may explain what is under that cover in the Windstar Forum. The trans are quite similar, so it should be relevent. I don't what other vehicles share the AXOD/... trans. It kind of sounds like either the reverse valve, solenoid or band, but I'm not very familiar with the intricacies of transmissions.