A 5.8 gasser with 4.10's and ZF was rated to tow 12,500 lbs on the bumper , 1991 cclb 4x4 if I remember correctly. I am aassuming that my idi with an ATS turbo can accomplish the same. There are many aspects that go into rating a truck; can they get to a safe merging speed in a certain distance, stopping, starting on grades, reversing on grades. I believe the standard for a merging lane is in the 600' arena. If you can only accomplish 35 mph in that length, you are a safety hazard. I read an article that went into great detail about this very matter, can't remember where, maybe Diesel Power??
I have tested the limits at 28,000 lbs gross wt. Yeah, I can do it, but not real well. Had to put T-case in low range to start on a grade, otherwise not too bad around town. Definitely would not try that on a highway though.