Did you actually take the diff apart to add the grease? Do the diffs work ?(i.e. when you turn a wheel, the opposite side turns in reverse)
I could be wrong, but sounds to me like a case of Durga-diff. Due to the fact they are half plastic, if they aren't assembled just right, durga diffs can create so much heat they melt the pulley. Has happened to me twice. One time the diff completely fell apart, the other time, it looked fine till i took it apart.
To make sure it can't happen again (if it is that, or even if not, this is a good idea) get a set of metal diff halves, either the 501x part or the 3Racing one, and a set of ceramic diff-balls. It's mainly the rear that needs it, but if you can afford to do both, do.
Only other possibility I can think of... you didn't put grease in your slipper did you?!
