it has to go through the clutch to get to the wheels, i understand where you are coming from, but think reverse from the wheels to the clutch, if your tires are getting good traction, what then is the weak link, if your clutch is slipping, the clutch or clutch system is the week link. so in my case i did not have but stock trans and transfer-case and a stock engine, and i also had a friend with an 88 stock truck including tires, my tires were 30x9.50 15, and he had the same problem on steep hill climbs, miner if you had weight in the back.
