Can't help with the numbers but....one thing is certain. They will be rated for MORE than the axles on your truck so unless you're greatly overloading the axles you won't have a problem.
Anyone know a good source to find out. I was at a tire dealer that suggested the 4142 number was it. They also called the dealer who told them it was 8600. I could call the dealer myself, probably will to see what they say.
Doesn't really matter though. Your current tires already support more weight that your axle can. Most likely your rear axle is rated at 6k lbs. Higher weighted tires won't do much for ya.