Driveline vibration out of now where! Help!

A couple simple next steps:

Drop the drive off the rear end, check the U-Joint at the same time. Then roll the Pinion Shaft Yoke over by hand, if there are bearing or differential problems in the rear end you should be able to feel it right here.

Also turn the passenger side brake drum/axle and see if you can feel any bearing roughness here, do the same with the driver's side. Good time while turning the brake drums to make sure the opposite side is turning in the same direction telling you the Sure-Grip is still engaged. If brake drums are turning in opposite directions then there is a problem with the Sure-Grip wore out or broke.

Next in line internally in the center Chunk, Pinion bearings and Carrier Bearings. These both get rust pitted on the used rear ends. If it was sitting for long periods of time it will rust pit on just one area of the ring gear possibly causing that rumble strip vibration.

Carrier bearings for the differential carrier can wear and rust, opening up the distance between the Rings Gear and Pinion Gear as much as .010" ths. causing things to make noise from misalignment. You should be able to feel this by rocking the pinion yoke back and forth by hand and listen for the clicks when pinion engages the ring gear. This is supposed to be like .008 to 012" ths.

Then finally the Sure-Grip, 4 spider gears in there with their cross shaft so something could go wrong here. And the Clutch Packs for the Sure-Grip too. Also there are supposed to be buttons on each side of the Sure-Grip where the axles bottom out against when you are setting the axle end play on the passenger side. They can sometimes drop out and fall into the bottom of the rear housing when pulling the axles. If you have the Green Axle bearings then you don't use the buttons, no end play to set.