Try to spin the wheels and hold the center shaft. Or hold the truck by the rear wheels and try to roll it on its front wheels. YOu should feel it when the diffs are damaged.
Shadow, are you brushless and running a completely tightened slipper?
I had to rebuild the diffs on both maxx's after they started making a "popping" noise...and I couldn't acheive top speed (slippage). I believe this was a result of brushless power, stock slipper tightened all the way down (i.e., no slippage), and a short track w/ lots of on/off the gas throttle action.
I replaced a couple gears, shimmed the diffs, and am looking to order either the Strobe or the RRP slippers.
Not 100% certain that's your problem, but it sounds similar.