Mine would start right around 60 mph and get worse the faster I went. I tried everything including balancing the drive shaft. I too have a 12 bolt, so I opened it up, removed the c-clips and brought both axle shafts in to a drive line shop. They spun it with a dial gauge right on where the bearing normally sits. Turns out the passenger side was WAY out of spec. Bought a new axle and the problem is now gone.