Well, for tires and wheels, you could bash a "Mad Mudder" that is in production.....it has the correct tires and most of the driveline is usable too. For the engine, I would use a nice resin unit available from Detail Masters. I would use them for plumbing and such also. For the tranny, I have seen resin transmissions out there.....somewhere..... As for the body and chassis, the body can be anything and the chassis is usually based on a production chassis, but highly modified......some are also totally scratchbuilt from simple rails and severely reinforced.... they're all different. Hit the net for detailed info on Pulling Truck chassis's.
I hope this helps a bit
Good Luck!!!!
Ken