It's detailed under "Nitro Rustler caster block & spindle conversion" here:
http://www.ultimatetraxxas.com/rustler_phase3.html
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