2004 Supercharged XJR, runs great for the most part. Having intermittent issues with the TCM dropping into limp mode, but should soon be rectified. What's puzzling is that when I'm running normally down the road and there's any sort of incline/hill need additional throttle to maintain speed, there's a low hum and sometimes intense vibration that develops in the rear of the car. If I get on the accelerator aggressively, it goes away, or if I get off the accelerator, the same. Thoughts anyone?

 

Could be a deteriorating propshaft flexible Jurid coupling allowing some irregular drive shaft movement under certain loading.
That said you might also have some bushings in the rear suspension that are allowing rear wheel misalignment to occur.

Good luck with the hunt.

 

Thanks Mark. I'll check those out.

Cheers!

 

Hope all is well.........Any update on tracking down your vibration?

 

Sorry for the late reply - haven't had an opportunity to get at it just yet. Work . . .