Look at your rpm's when you do this. If you let off the gas and the r's drop pretty good, then when you step on the gas and the r's go up further than when you let off the gas (say, you're doing 50'ish and the r's are at 1500, but when you hit the gas again they jump up to 2000), then it's your transmission downshifting when you hit the gas (or the torque converter unlocking and allowing the higher rpm's).
If the rpm's go back to where they were (1500), then it's drivetrain slop (trans, t-case, rear diff, rear driveshaft [front u-joint or rear cv] or rear diff). You can check the rear driveshaft by shutting the engine off while in neutral, applying your parking brake (hard) and then crawling underneath the truck. Grab your shaft (insert joke here) and the rear diff yoke and twist them in opposite directions. If the shaft moves, the rear cv is going out. Do the same thing with the slip yoke at the t-case, trying to move them in opposite directions. If there's movement, then the u-joint is bad. If both are good, then try to move the shaft sideways, up, down and even in and out of the t-case to see if there's any slop. If you can't find anything with those tests, then it's more than likely just normal slop either in the trans or the rear diff. If there's no howling or gravel noise coming from anywhere, drive it and be assured you checked it out pretty good. If there is noise anywhere, doesn't change with gear selection and gets incrementally louder with road speed, not engine speed, then your rear diff needs checked out.
