It's all that extra HP tearing up your rear-end . It might be a failing universal joint. While sitting still, foot on brake, shift from D to R & back again while listening for a clank. You might also try that while somebody is watching the drive shaft. Compare movement between front & rear joints. If one moves more than the other, then I'd replace. 84 & 85 vans have replaceable joints, so at least there's that.

This could also be caused by a harsh shifting problem due to worn or shrinking check balls inside the transmission valve body. Here's a thread where this was recently discussed: http://www.toyotavantech.com/forum/s...1370#post11370