Those things are really complicated and there are too many things to start guessing at. So your best bet is to have a good transmission shop put it onto the right kind of scanner.
A few things I would do just b/c they are easy & cost nothing - there is a ground strap for the transmission - it connects to one of the bell housing bolts in the front. Pull that off, check the whole wire, clean it, reinstall. Pull all of the electrical connectors, blow them out with electronics cleaner, finish with a dab of dialectric grease.
There are two speed sensors - one input & one output. They just thread in & out. Those can be pulled & checked for metal. Here is a diagram: http://dodgedynasty.50megs.com/a604/basic.html
Of course, this is just tinkering. It probably won’t help a thing (but it might!) So if you really want to know it has to go on a scanner.