If you think it's the transmission is original to the truck, replace it. '84 is a bad year, all the early ones are built with weak parts. Later years they refined the design of a lot of parts inside to be stronger.
The best 700 I've ever been near was a fresh rebuild to factory specs, which is pretty much any of them built professionally with any kind of warranty. What made it so good was the
TransGo SK700 kit installed in it. This is not a high performance reprogramming kit, it's a bug fix kit. It fixes all the common complaints, and some you maybe never noticed until they were gone. If you've got by some mistake or misfortune the "554" servo piston, replace it with the
TransGo 7-2P.
I know this sounds like a commercial, but trust me. The guy at TransGo knows whats going on. On a related note, the 4L60E kit is gold too.