Help with driveline vibration

I have a request for help.

Recently, my 2010 V6 double cab 4x4 with approximately 59,000 miles on the odometer and 10,000 miles on the tires, has developed an annoying problem over the pass few thousand miles.

At speeds above 55mph the truck has a nasty vibration in the rear. The vibration can be felt in the front and rear seats as well as the floor. At speeds below 55mph, no problems. No vibration is present in the steering wheel at any speed.

I took the truck to the local Toyota dealer and purchased an alignment, rotation and balance. The dealer actually, refused to complete the alignment, telling me the alignment was spot on.

Even with the rotation and balance the damn vibration persists.

I crawled under the truck and looked at the u-joints and found the u-joints on the rear drive shafts are spraying grease on the belly of the truck. Basically, its a perfect circular spray of grease.

Additionally, at 55mph all the way up to 90mph if I shift the transmission into N the truck suddenly feels as smooth as silk.

Is it possible the u-joints are going bad or are bad and causing one hell of a mechanical vibration in the driveline that is felt in the seats and floor?

Any help or direction is greatly appreciated.

Thanks!

 

best way to isolate this

disconnect rear driveshaft

drive around in front wheel drive, does it still vibrate or not