I got the cen gst diffs off ebay. They are a popular upgrade for savages and they also work in the hellfire. The pinion and crown fit the hellfire plastic cups with some shiming. They have alot stronger teeth.
I would just initally check/shim your diffs, upgrade your servos (if they are the original servos they wont last long) and keep an eye on that mid range needle. It will fall out if it becomes loose.
Also check all the grub screws in the drive train as they come loose and fall out. They need locktite.
Check the hellfire thread on truggy racers.com. It has alot of good info just like rcu.