Thanks for the ideas. I just got in from running it on axle stands and I think it might be a differential bearing. Everything seems to be running straight, wheels, tires, etc. and the guys that checked the tires said the wheel bearings were good. Also, that would explain why it happened with the rear driveshaft out and why it's worse under load.

When I let it run on stands with both wheels turning and tightened up one side of the e-brake, it made a light growling sound as the power switched to the free wheel. Even with the motor off and in netral, There's quite a bit of rotational 'slop' in the pinion before the wheels start to turn. Any thoughts? The rear end has always been a bit noisy but could an out-of-whack bearing create the vibration? It's pretty harsh around 25-30 mph and seems to be getting gradually worse.

What's a diff rebuild worth?