Ramcharger vibrates after using 4wd

I spent some time on it today. Used the FSM and went through with operation the CAD. Spun the tires, front shaft stayed put. Put a vacuum pump on it and shaft turns with tires. Disengaged easily. Pulled the automatic hubs apart and nothing sticks.
Pulled the rear driveshaft and checked the ujoints. Smooth as silk, no binding. I installed them in the spring. Pulled the wheels. Brakes are not dragging. I did not change the trans mounts. They don't look bad.
With it up on the stands, I ran it to 65mph and the vibration is still there. Starting about 45mph. I presume my next step is to pull the rear cover and check the differential. I find it ironic that the vibration started after using the 4wd. Could the front driveshaft, even though it doesn't spin in 2wd, transmit harmonics through the floor? Its gotta be something simple, I'm just not finding it.