transmission/service engine light

I have a 1998 Pontiac Trans Sport with around 72,000. The transmission went and I had it rebuilt last year. Everything was fine until it started to slip again. I took back to the shop and they dropped the pan and there wasn't anything in the pan, metal, etc. When I'm driving and slow down for traffic, etc., sometimes it seems to slip out of gear and the car just reves until I put it into neutral or park and then back into drive then it goes into gear fine. I was told it could be a sensor??? My service engine light just came on today. Does anybody have any ideas?

find out what the SES light is first off. Is the transmission still under warranty if so go back and complain some more. The trans should no fall out of gear.

My brother pulled the code this evening and it is P0172.