OK, the moment you've all been patiently waiting for. I thought I had mentioned this in a previous post here but I don't see it here so I guess not. I definitely had discussed it over text with some of you Rover aficionados. When swapping driveshafts around I noticed that the trans mounts were not square. The nut on the driver's side mount was in the middle and the passenger side was wedged firmly against the rear of the slot in the bracket. This means the output flanges of the tc were not parallel to the input flanges of the diffs. I guess for the rear this isn't as big a deal because of the guibo, but that massive double cardan joint on the front driveshaft did not like being rotated at an angle.

Tonight I finally got under the truck again, undid the two securing nuts in the trans mounts and jacked the trans up. I used a pry bar to jimmy the trans about a little bit, lowered the trans and put the nuts back on. This time the two nuts were as you see below, neither were in the middle but they are each in the same position relative to the slot itself. The vibration is gone.

 

Great that you found the root cause and thanks for updating. I have a slight vibration around 50-70 mph too, I will check my mounts now. I know that the drivers side engine mount is cracked, maybe that engine vibration brings it out of alignment.

 

The irony here being that the shop that installed the transmission and noted the vibration never bothered to check to see if they caused it through their own carelessness. Amateurs, Dude.