They're German axles. ZF I think.
Our government ordered Dually single cab C30's (2wd) with no beds, sent them to a company in Germany, and they converted them into those tugs. Planetary axles (similar to a Mog but BEEEEEGER), DEEP gearing, I *THINK* rear steer, full time transfer case...can't remember if the t-case was reduction or 1:1, chopping off part of the rear frame, and the suspension consisted of large, heavy rubber blocks between the axles and the frame. Top speed was damn slow.
I worked at a FBO at the Gerald R Ford International Airport in Grand Rapids, MI a few years back. We were starting to get customers with larger jets (over 50k pounds) and my boss traded for one of those. Our mechanic had it working pretty well when I left that job. I've since seen a few converted into mud trucks by ditching the bumpers and swapping some GM parts back in for the transfer case and suspension.