RE: Center Diff Locked up
The diff cases wear out over time from the pounding, sudden jarring (especially from on power landings). As you change your oil in the diffs during routine maintenance, check over the diff housing. If the slot the gears/shaft slide into, replace it. It's cheaper to replace the plastic housing than the plastic housing and the messed up gears.
I dunno how much power your motor puts on, but I run a monster brushless setup that has instant power and I've noticed I need to keep an eye on the diff housing.