I have a 00 expy, and occasionally the transmission will go absolutely beserk, such as not engage at all, or slip in low and reverse, fine in high range. Or it will make a noise in low and reverse (sometimes not always). It may not engage at all, then a few minutes later it may. Or it may be fine for months, then it may start all over again. It shifts into 4wd and low4wd fine. Any one with the same problem?

 

First off how's your fluid level? Slow shifting, or not shifting at all are symptoms of low fluid. The 4WD shifting you mentioned is not related to transmission shifting, it's more of a mechanical function. If your fluid level is fine, you may try some cleaning fluid, or changing fluid along with the filter (cheaper fix). You wont remove all of the old, but if you experience improvement, you may consider a transmission flush and change at a shop.