My XJ starts to get a vib at 55mph and worse as I speed up. I had it jacked up and put in gear and as soon as I let off the gas it has a clunk noise. But when I'm driving down the highway it sounds like its growling anyone have a suggestion on this.

 

Specs on lift/no lift D44/D35

 

No lift and i believe it's a 35.

 

Yoke bearing, u joints or worse (gears). Where does the clunk come from? A growl, the first thing I would check is the yoke bearing. A clunk sounds u jointish. Unbolt the rear driveshaft and try to wiggle the yoke going into rear end. If that's not it, check your u joints while you have the driveshaft out. XJ's are funny in that under load I have seen shot u joints seem to be good. Only way to really check is while driveshaft is out.