I have a 1990 GMC Sierra Sle with an automatic tranny. When I put it into gear the truck lurches foward and makes a clunking noise. I'm wondering why it does this. If anyone knows please help me out.

As far as jumping forward, what rpm is your truck running at when you put it in gear, if the rpms are too high then it will jump forward when put in gear. The clunking, check to see if you have a u-joint that has gone bad, you can crawl under your truck and grab hold of your drive line and twist back and forth to see if there is movement in the u-joint. If the movement is in the rear diff then it is probably the ring and pinion gears are wore out. Hope this helps.