I have an 97 with an automotic and I have had it rebuilt. I also run 4.88's but with 33's. It has been 10,000 miles with few problems. These trannys do tend to leak fluid and this could be causing the slip. I have had to tighten the bolts on the pan twice, it was very noticible (the slip as well as the leak). If your fluid is good (also make sure that they put the right weight tranny fluid) then I would take it back. If the guy is lagit he should fix it for free!!!