The stock bearings are heavily greased and they also happen to be open or non-shielded bearings. The problem with the grease is that it is so thick it actually slows the bearings down and dirt also sticks to it which will cause the bearings to seize. So, the degreasing of the stock bearings provides to advantages: the bearings will spin much faster (your car will be faster) and, two, they will be much less likely to seize up.

cool ill grab some stp at work

I just realized I misspoke in my last post. When I referred to the "stock bearings," I meant the RadioShack XMODS upgrade bearings--as opposed to an aftermarket set from some other company. There are actually no stock bearings in the XMODS. The XMODS come with plastic bushings.

Re: Vaseline?
 

Don't use vaseline. If it gets cold it will harden and mess your car up. I didn't use the grease either because anywhere you race, you're bound to have dust or dirt. And grease likes to attract dust and dirt, thus screwing up your gears.

But there shouldn't be gears in the ball differentials for the AWD kits (I don't own one though). Inside the differentials there are very small planetary gears or balls that kind bind up pretty easily, and they are not exposed like a pinion gear would be. Plus the grease can slow down the action if you want.

its ok to grease the differentials but make sure to put a little in the ends to attract any dirt/dust so it doesnt get inside the diff.

the diff. that comes in the awd kit is ball bearing