It could be a few things here is a list of the area I would check
1. Rear shock and springs, make sure they are in the same oil wt and spring rate, check the bladder inside the shock too!
2. Check the rear diff, if it is too loose then when power apply to the rear axle and both rear wheels are trying to fight for traction and if they dont spin at the same speed then you will have "Wriggling" types of react!
Hope that helps