Neutral eliminates basically everything but axles, rear suspension, and tires.... You take all the load off the trans and tcase, and essentially driveshaft, when you goninto neutral.... Now where exactly is the vibration, the seat or the steering wheel....

 

Seat and wheel. My side view mirrors shake too.

A ds still can vibrate without load, right?

I got a friend with a proper lift that said I could test on to high speeds.

 

Seat and wheel is tires/rims.... Unless something is coming apart you wont TYPICALLY feel a rear vibration in the steering wheel... Id be looking hard at tires and rims, being on a lift is gonna cause a whole new set of vibrations....

 

roadforcecoughroadforce

 

Cough hell... He needs to get back home do not stop, do not pass go, go straight to tire shop.. And get the damn wheels balanced, road force preferred but at this pointthe tires need to be spun on a machine and someone look at the tires and make sure none are out of round or flat spotted or whatever.... Sadly he probably had this vibration before the trans swap, but after driving that jap rod hes got, forgot about it and now trying to chase it down.... Do believe i mentioned this to him months ago....

 

Tire vibration feels like the whole truck is shaking. Driveline vibration is a much higher frequency, more audible, and feels like you have an orbital sander under your truck. 65 mph is about the oscillation frequency of an off-balance 30.5" tire. For a smaller tire the MPH will be lower and for a larger tire the MPH is higher.