My best guess is that he may need to use heavier diff oil so that the diffs don't unload as much, or he may need to shim the diffs for a better gear mesh, if the gear mesh inside the diff is too tight there will be a lot of friction, which will result in lots of heat. Maybe somebody with more experience could chime in but that's the only thing I can think of, unless there are binds in the drivetrain. He may need to use heavier oil too? Good luck finding a solution. I haven't heard of anything like this before so I'm thinking it has something to do with how he has his trucks set up.