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Did any part of the recent work include any suspension changes that might affect the driveshaft angle?

With 170K miles on the driveline, have the U-joints on the rear driveshaft every been replaced?

How about the service history for U-joint lubrication? Lubed at every oil change since the vehicle was new?

There will be no value in trying to balance the driveshaft if there is any detectable wear in the U-joints, and driveshaft vibration caused by worn U-joints can create the low-frequency shake you describe.
 

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If you are saying that your suspension mods lifted the chassis at front and rear another 1.5 - 2 inches, then your driveshaft angle definitely increased (this affects the range of angular travel that the U-joint needle bearings swing through for each revolution of the driveshaft).

The shop that did your re-gearing ... they are experts at assembling differentials and setting up differential bearing preloads, and have done scores or hundreds of diff rebuilds?

The new "humming" noise and the new vibration could possibly be linked; neither are normal after a diff overhaul.
 

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If you've got adjustable rear links, you can 'roll' the entire axle a few degrees to get back closer to the previous driveshaft angle and see if that makes any detectable change in the vibration. Even if there is some level of wear/clearance in the U-joints (and there has to be after 170K miles) reducing the driveshaft angle should make some small change in vibration, IF the vibration is U-joint related.
 
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