I was thinking exhaust too.

 

I had a 91 silvereado and it had the same problem. It turned out to be the tranny mount where the tranny sits on the crossmember, the rubber rotted away. The way I found this out is the truck was off and in park and I got behind the truck and pushed it forward and pulled it back.

 

ok thanks, ill crawl under it and look to see if it might be the exhaust or the mount. i know that i have a recall on my truck for exhaust clearance problems.