Welcome to the site! Now with you diff problem. I am not aware of any after market metal gears for the xxxt. The reason you diff is melting due to it being set to loose. I would get a new plastice gear, diff balls and thrust bearing set and rebuild the diff. I would reccomend Bfastrc diff balls, diff rings and thrust bearing set. I have been running my set diff parts for over 4 months with a Mamba Max 4600 and 7.4v lipo and it still fills like the day I built it.
Here is a write up from the Bfastrc thread on how to rebuild and break in the diff.
B-FAST R/C PERFORMANCE
Differential Break-In Procedure
1)Clean all parts and work area
2)Follow owners manual for assembly instructions
3)Lightly Coat Diff Balls with Silicone Diff lube, thoroughly lube Thrust Bearing with AE or Losi BLACK grease. (or equivalent)
4)Snug diff screw just until gear will not turn while out drives are held.
5)Assemble transmission per manual
6)With assembled transmission in car and car on a stand, hold one rear tire while lightly applying the throttle for about 20 seconds (about 1/4 throttle, No tire swell). Repeat while holding opposite tire.
7)Tighten diff screw about 1/16 of a turn and repeat step 6
8)Repeat step 7 two more times.
9)Slowly loosen diff screw a little at a time so rear tire turns with little or no resistance while holding the opposite tire.
10)Slowly re-tighten diff screw a little at a time until resistance is felt while turning one tire and holding the opposite.
Drive car and adjust diff to suit your driving style.
Do Not let the Diff Slip at anytime!!