Trans issue on the Gussler Van?

Does it do it in manual low as well?
How about second gear at very low rpm?

The trans mount is operating in compression right? With the rubber underneath the trans, and then the cross-member. Are the engine brackets bolted onto the engine in the usual place?
Also check your rear springs for broken leafs, your U-joints for dryness, the slip-joint for adequate clearance into the trans,and the U-bolt torque.
And finally, make sure the idle air blades are clear and not causing a rough engine operation, in the low-speed circuit.