i used MMO for years had my picco sitting in the garage for 6 years and I treated it with MMO before I put it up, fired right up the last time I ran it.
http://www.ebay.com/itm/XTM-Racing-X-Factor-x-factor-monster-truck-rockcrawler-tmaxx-/171656774230?pt=Radio_Control_Vehicles&hash=item27f78a8256
ends in an hour and some change
Get the engine completely broke in and tuned before worrying about shifting to second. Most of the 2 speed nitros I have didn't hit second until after the engine was broken in and tuned properly, don't rush the break in.
Yep a fast trigger finger will destroy diffs quickly, if running an overpowered vehicle roll into the trigger to get it rolling and then let it fly. Wheelies are cool but tough on the drive train.
Here goes my usual answer on this after you check what X has said. Next check your wheel hexes and see if one is stripped on the front end start with the left wheel then I would check my front diff. I doubt the diff is the problem based upon the symptoms you are giving but I could be wrong.