Have you checked the fluid level lately? As winter comes, and it gets colder, the fluid level tends to be lower in the morning than in warmer seasons of the year. Check it properly!

You may just need a fluid flush (go to IPD's website to see the instructions on using their fluid flush hose, even if you don't want to buy it and intend to make one yourself) -- fluid does wear out, and seriously affects shifting and other transmission operation.

These transmissions are generally bullet-proof -- it's hard to believe that you really need another one, unless you've really abused it: never changed the fluid, towed a trailer too big and cooked the fluid, manually shifted improperly, etc.

I would definitely do a thorough flush before I considered doing a transmission swap! Tranny techs are not always as smart or as honest as you might think.