I've inspected u joints all seem tight
Have brand new tie rods and ball joints
It seems the thud is rpm related but only when accelerating!
Consistetent thud doesn't sound engine related.
Any ideas where to start checking?

99 4.0 aw4 np231 re long arm lift sye lunchbox rear

 

You might check the splines on your slip yokes. If they're not greased, they can stick a little and thud.

 

Check your tranny mount, if the rubber is split it will thud when accelerating

 

As well as engine mounts, they should all be replaced at the same time

 

First guess was going to be dry slip yoke splines on the rear DS but since you listed a SYE that's not it. I guess it could be the slip joint sticking in the DC shafts, so I'd start by greasing those and see if it goes away.

When I first got my XJ I "inspected" the U-joints by grabbing them and trying to find any looseness or wiggle but they seemed tight. I had some pretty bad vibes and a clunk so I pulled the rear DS anyway and found the u-joints to be pretty bad even though they felt tight on the vehicle! One was dry, rusty and the spindles all had ripples worn into them from the worn roller bearings. Unless your driveshafts are brand new, I'd pull them and give them a more thorough inspection if greasing the slip joints didn't make the noise go away.

Like others mentioned before though, also check the motor and tranny mounts - those could easily be the problem as well! (and you do want to do those both at the same time, unless you like doing things twice)

 

Okay some great help
Now just hoping to work less than 12 hrs today so I can check it in some day light

 

Okay some great help
Now just hoping to work less than 12 hrs today so I can check it in some day light

 

Turns out my driveshaft had been cracked causing my ujoints to clunk. Finally gave way at 50 mph on the freeway..

 

Bummer buck, need a new one? PM me ill be in aptos tomorrow