Have you checked the tranny fluid level, and have you ever had a transmission system flush like BG? Many transmission places are willing to go for a test drive for free, you can get there 2 cents, then decide if it may be a more involved issue\cost.
I had a ranger several years back that developed some minor shift slipping (no funny noise), and the flush and fill almost took the slip completely away.
Best of luck. Doing any other tranny maintenance yourself without that specialized knowledge could be tough. Oh, have you checked to see if there is a Technical Service Bulletin (on tranny issues) for your year, make and model? I tend to check those first when a new problem arises, just to see if it is similar to one already reported and find out what the possible fix might entail...