Suspension or Transmission faults -Series 2a

I've got a Land Rover Series 2a and the problem I have got is when I pull away in First or Reverse, the axle or transmission is jumping. I have poly bushed the rear axle and shockers to remedy this but it is still there and its geting worse. Any ideas? I haven't had it for long but the prop shafts look new.

rear diff breaking up?
Handbrake sticking?

The handbrake is fine and the rear diff is meant to be new he ran it on a rover p6 diff to go on the motorway before he sold it to me