Anybody ever transform a 4WD rc truck into a mudder truck?

Personally, no, but I've seen a lot of people convert HPI wheely kings and Tamiya Clod busters. Basically you need a 4wd truck with good ground clearance and tires(usually chevrons or swampers).It helps to add rubber shielded bearings at the outer positions to keep it sealed better; also a bead of grease at the seams of the axle and trans cases. Lock at least the rear diff, too.