Jerking when taking off would not be a rear diff unless you are spinning one the wheels first then the clutch kit locks and the other tire grabs. If the diff had the wrong fliud you would feel a vibration/slippage when making tight turns. So if the truck doesn't have any vibration or slippage when making a tight turn you can rule that out.
Other things to check other than the Rear Diff would be tran fluid which you said is at a good level. Be sure the right fluid is in the trany, but if you haven't changed it, it most likely is the right fluid.
If the slippage is during takeoff I would suspect the torque converter. When the slippage happens does the RPM's go up then back down when the truck hooks up?
You may have prematurely wore out the trany with those oversized tires.
Let us know what you find, I'm interested to know..
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Dogbyte
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