get this sttuff:
http://www.badhorsie.com/products/diff-lock-grease.html

Im using it in my tmaxx, and I love it, the grease is ridicously thick, but it stills allows you to turn the diff by hand, its great stuff because it doesnt handle like crap when you completly lock up the front and rear diffs, it wil handle horribly.  what im doing with my earthquake is locking the center diff with Jb weld, and putting that grease in the front/rear diff.