Rear end hopping.

what you are experiencing is spring wrap. your 289, depending on mods, makes are 250 or so hp, and with the 3.55 gears you have a good torque multiplication effect going. all that conspires to twist the rear springs rather severely, and when the springs untwist, it actually lifts a tire off the ground and what you feel is the tire actually bouncing. what you need is a set of traction bars to eliminate the spring wrap. either caltracs, or traction masters bars are the ones to use as they dont change pinion angles like ladder bars do, so they work well in all driving situations.

rbohm said it all.

Sounds like axle wrap...it's a common problem...traction bars will be a big help.