97 Jetta Tranny Problems

I have a '97 Jetta that is slipping gears (BAD!!) I took a look at the trans fluid and Replaced the filter and all the fluid. So I know for a fact that the fluid is good. Which leads me to believe that the tranny is bad, or needs rebuilt. I have been looking at rebuilt/second-hand tranny's and seems to be the cheaper route.
My question is: Where do I have to look to find my tranny code?
Also what other parts will I need to replace during the swap?