If you have ever used M/T tires, you will have noticed a vibration due to
the tires at very very low speeds (1-2)mph.
Driveshaft vibrations will feel almost the same, but in higher speeds (~10
mph or above).  Because the feel is similar, if you do the lift and switch
to M/T tires at the same time, you won't be able to tell for sure if you
have driveshaft vibrations, unless they are very severe, or you know exactly
what to feel for.

Incorrect pinion angles, will have as a result the driveshaft to increase -
decrease rotation speed, twice per driveshaft turn, or 2*[axle ratio] times
per wheel revolution.
For example, if you have 4.10:1 gears, the driveshaft will pulse 8.2 times
per wheel revolution.

Since the wheels rotate with almost constant speed, the fluctuations of the
driveshaft speed are transmitted to the clutch and crankshaft of the engine,
via the gearbox.
So this will give you the impression that the engine is not running
smoothly.

Besides that, it transmits a vibration to the entire car body, which is
proportional in frequency with vehicle speed, and in strength with the power
applied to the wheels.
It may feel as one or two cylinders are constantly not firing.  But as speed
increases it may be heard as a low pitch whine (lower pitch than the whine
you get from the differential gears or the transmission or the tires).

You may have to experiment with the pinion angles to find the angle that
makes the engine feel smoother running on all driving conditions (cruising,
acceleration, compression braking).

Bill Spiliotopoulos,
'96 XJ,
'06 TJ.

Yes, just before it pole-vaults the car and you cart wheel down the
road.

** Posted from http://www.teranews.com **

sounds like wheel bearing or badly cupped tires
my bet is its tires esp. if they are bf Goodrich's triple wall tire