No solenoids working on a 700R4 at either point you mentioned in their shift chronology. Have you checked the fluid? When was the last service done? Any strange noises? Have you adjusted the TV cable at any point since you’ve had it? Do you know how many miles are on it since the rebuild if there was one? I can’t think of anything external that it could be. I’m worried about the low-reverse clutch pack in this situation. You’d need to pull the pan and check for particles. You can also pull the filter, cut it open, and check in there. Might as well go ahead and do a service if the filter is old and the fluid is brownish or kind of dead looking. If it’s new, it’ll be hard to tell anything, and if it’s black, the transmission is done.
Maybe someone else will have other ideas, but report back with the basics, and after that, drop the pan to see what’s going on. It might just be time for a rebuild, but there’s homework to be done before officially reaching that conclusion.
