If it isn't the tires, then I'd definitely check out the joints on the driveshaft.
I drove a '84 oldsmobile, rwd one time... between 55-60mph or so it would start shaking like the devil... then it'd be fine... my pop crawled under there and the joint on the driveshaft towards the rear going into the rear axle was all beat up - it was just slap worn out. Replaced it, went back to driving it, no more problems.
It is something to check anyway,
-W
