I have a Firehammer which is virtually identical to your truck. On the diff, once you get it apart look for metal shavings in the grease and ground down teeth on the bevel gears inside the diff. My plastic diff housing broke and I didn't catch it in time to save the bevel gears. The case is inexpensive but the gears are not. You can use Duratrax Firehammer/Firehammer MT parts on that truck. Smartech makes these clones (Duratrax, XTM Racing, Carson) as Foxy said. I run an FG alloy diff housing and it fits perfectly on a Firehammer. The replacement FG alloy case/sleeve runs about $60. The plastic one will hold up if you don't bash hard.
Any local hobby shop that sells Duratrax can get you the parts. You can also get everything you need from Tower Hobbies (
www.towerhobbies.com).
The full parts listing on Tower can be found here:
http://www3.towerhobbies.com/cgi-bin...I=DTXD98**&P=Z
The plastic diff case can be found here (case and sleeve) for $4.50. You local shop could get it for $5 - $6 and save you the shipping charges.
http://www3.towerhobbies.com/cgi-bin...?&I=LXHTU4&P=Z
After cleaning all the good diff parts off good you will need to re-grease the diff with heavy gear grease. I used Lucas Red'N'Tacky gear grease but I have heard that ball-joint grease will also work. Use just a liberal amount of grease on the gear faces. It is not necessary to pack the whole diff with grease.
If you can find the budget to do it I would highly recommend replacing the small plastic pinion gear on the clutch bell with a metal one. This gear heats up easily, softens and the teeth strip off. I went through many of these gears before finally getting the metal one. The larger plastic spur gear works great with the metal pinion gear. You can get a metal spur gear at Large Scale RC for about $23:
http://www.innovativehobby.com/index...OD&ProdID=1015
If you need to run your plastic gear for a while then try to run it a little easy to keep the heat down. You might want to order a spare plastic pinion to have on hand.
Lastly, you can download a great exploded diagram of the truck from Duratrax or XTM. Here's Duratrax's diagram. The first page has the diff breakdown on it:
http://downloads.hobbico.com/evpl/dt...8-exploded.pdf