definately slipper or diff.
Brushless generates so much more torque it would make a brushed setup not work.
Did that with my stock car and popped a brushless in and it wouldnt launch just accellerate gradually. Tightened up teh diff and bingo it was launching.
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0