got a hop! 4x4 issue moved from 88-98 forum

[highperf4x4]

Hey guys, I have a question about my suspension that someone might be able to help me with. About 2 months ago, my truck started "hopping" when I hit a bump around 35-40 mph. It does it really fast and I have to slow down to make it stop. Sometimes it almost feels like it's bouncing side to side like a twisting motion and it's so severe that I can't tell if it's coming from the front or the rear. I thought it was a loose wheel so I checked them all and they were tight. I crawled underneath and looked for something amiss but couldn't really see anything other than a shock leaking on the rear. I replaced all 4 shocks just to be sure but it's still doing it. The truck is a 1990 GMC 4x4 1/2 ton. I'm running 16" Eagle alloys and 265/75/16 tires. The wheels have been on the truck for several years now and the tires for about 1.5 with approx. 40% tread left. Someone suggested the front torsion bar but all the grommets seem to be intact. Anybody else have this problem or a suggestion on what I might check next?