I think this will work well... A winch motor, and robot speed controller...
This motor:
http://cgi.ebay.com/ebaymotors/NEW-MOTOR-WARN-WINCH-12V-KEYED-SHAFT-HEAVY-DUTY-8274_W0QQcmdZViewItemQQhashZitem35a60c1374QQitemZ230419075956QQptZOtherQ5fVehicleQ5fParts
Winch Specs...