Under normal circumstances, I'd say your pinion angle is off...
HOWEVER, if you're getting a vibration accelerating AND deaccelerating, it can't be pinion angle since it can't be off both ways.
You'll either get a vibration stepping on the gas or letting off the gas.
(not to mention that though our trucks are AWD and the frt driveshaft is turning all the time, pinion angles are not quite as impactive on the front driveshaft as it is on the rear)
Before I did anything, I'd swap the driveshaft 180 degrees. If that doesn't help, I'd say check the Ujoints and have the driveshaft rebalanced.
I doubt that your vibration has anything to do with tires for a couple of reasons.
First, unless you didn't recognize the vibration before, the tires haven't changed so the vibration shouldn't have just started... if your tires are the cause, it should have felt the vibration before you had the diff replaced.
and second, tires out of balance really won't show a vibration at 20-30 mph (under most circumstances for tires these sizes) Vibrations from out of balance tires usually start 50 mph and up (normally)
Last but not least, there is a minute chance that if the gears are setup incorrectly, it could cause a small vibration (too much backlash, wrong depth, etc, etc).. but like I said, possible but highly improbable.
My .02 worth.
Mark
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