[davepl]

Well clearly it should still drive properly without shocks if the factory offered them without shocks.

Are you certain you do not have 9/16" wheels on 1/2" studs?

After its shaken at least once, bring it home, lift it up, put a dial indicator on something like the bead, and rotate the wheel. I don't know what minimum vertical runout should be, but it shouldn't be a lot. That'll tell you if it's the wheel.

There are other possibilities like driveshaft critical speed but unless you have serious gearing I don't think you'd get there at that road speed. I'd still give the U-joints and carrier bearing a once-over just to be sure. I've never had a carrier bearing fail but if the driveshaft starts dancing it'd feel pretty wild.

What about the fronts? Sounds like you're confident it's the rear, but it's not always the easiest to tell.

I hated the Turbine wheels on my '75 Monte so bought a set of Superior wheels that were some horrible universal design. Not only was the hub center bigger than my hub, but the lug holes were elongated to fit anything as well. Stock rocks.