Rear end question

I have a 1999 Extreme. When accelerating or shifting (manual) the back end of the truck seems to move side to side almost like fishtailing (tires are not spinning). Very minor movement but I am just wondering what could be causing this issue. Are there bushings that could be going bad on the differential mount? I am actually in the process of selling the truck and a potential buyer noticed the problem. I had not noticed it in the past. Any ideas?

J

Shackle bushings? U-bolts loose? broken springs? Have someone wiggle the truck while you watch the rear suspension.