Do you mean an aluminum transmission case? Here's one:
http://www.rc-monster.com/proddetail...ny20000&cat=29
I have never stripped a plastic diff gear when using that.
If you haven't seen it, here's Traxxas' steel diff gear:
http://www3.towerhobbies.com/cgi-bin...&I=LXSHA2&P=ML
And of course Mike's HARDENED diff gear with aluminum case:
http://www.rc-monster.com/proddetail...edediff&cat=29
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz