Several Ofna buggies come with a center differential that can have a plastic spur gear put on. Just for that they may be the easiest to convert. Here are some of them:
http://www2.towerhobbies.com/cgi-bin...?&I=LXAXV2&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXAXV1&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXGUS1&P=7
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