Vibration in 4WD after Shifting to 2WD

I get a distinct vibration at speeds over 35mph from what feels like the right front end of the vehicle after switching from 4WD into 2WD. This is in an '84 3/4ton Chevy Suburban with manual locking hubs. It usually stops after a while. My transfer case is full of fluid and everything else seems in order. I can hear the hubs unlocking I believe. It seems that the front driveline isn't full disengaging.
I seem to remember that there is a test to verify that the 4WD is engaged or disengaged that can be done by lifting the front end of the ground and turning the wheels - but I can't remember exactly what it is.
Does anyone know this test or have any other ideas?

Thanks in advance...