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Make sure that the noise isn't actually coming from a U joint. When U joint bearings dry up they will give a squeak that is often throttle dependent and worse when accelerating and decelerating. You can remove the driveshaft and feel the joints by hand, if there is any binding then the joint is failing.
It's possible that the noise could also be in the transfer case and appearing to be coming from the transmission.
If the noise is coming from the trans for sure and the fluid level is full then you likely have a bearing failure inside the trans. It's also possible that there could be a spline wear issue between the trans and transfer case, either with the female splines on the transfer case input shaft or trans output shaft. If you're positive that the noise is coming from the back of the transmission area then I would pull the transfer case out and check the splines, if everything looks ok then the trans will either need to be disassembled and inspected or the unit replaced.