The aluminum/steel diff isn't necessary. You just need an aluminum idler. Switching to using these steel balls instead of diff pegs will make the slipper work better:
http://www2.towerhobbies.com/cgi-bin...?&I=LX2760&P=7
Also, upgrading to RPM bearing carriers with bearings will be a very good idea.
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