what is the center diff? is it the slipper setup.
Center ball diff. Great but not necessary... depending on your taste I guess. What is it explanation here http://www.rctech.net/forum/10015740-post6.html

what is the 8mm mod?
Drive the truck for now and get used to it while you catch up on reading this thread to see if 8mm mod would help you. You should be reading within the past month or two, use search in this thread or google's probably better than rctech search.

how should i set the stock slipper clutch? set it on the ground and hit the throtal, watch the front end to see how much it raises?
At least get the garodisks from RCshox for the old school two pad slipper for now if not the C-Diff. Adjust to where you slip for the first few feet @ full throttle on the track you are at... traction control. Start with manual suggested slipper nut setting and adjust from there for the traction at your track.

Shim rear diff...
Do you have a few shock limiter clips under the rubber bumper on the shock shafts? You need them. 8mm total including rubber bumper.
Call AE and they will mail thicker diff shims at no cost. I think .3 thick is correct, but the .5 AE ones cost $0 to your door and one minute on the phone.