Dodge cummings drivetrain issue?

Ok well my friend has a Dodge cummings diesle truck I'd say like mid 90's not the old style but not the new style. The style that had all the tranny issues. Here is my question he's got a bunch of upgrades to the truck it makes I believe 600hp and like 900lbs of tourqe. He runs a 3:55 gear in it and I would think that this gear is not right..shouldnt it be no less then 3:73's? He has gone through a bunch of tranny rebuilds and just yesterday blew his rear end. What is casuing his problems? I think he's making to much tourqe for the drivetrain to handle and when you make that much tourqe you can exspect to break **** all the time...unless you go aftermarket and beef the **** out of it.