8 to 6 lug spacer/adapters exist, but they're spendy, and 2-piece. No hubs are available to convert the military axle. You can buy a standard axle with 6-lug hubs and hydraulic brakes, but not with provisions for the mechanical parking brakes that the Mil axle has.
Might I suggest heading a different direction and change your 6-lug suburban to 8-lug.
The front is easy, you just need new 8-lug hubs. For the rear, I would look for a junkyard 14bolt semi-floater and toss that in. The 14 bolt semi-float axles are pretty easy to come by, reasonably priced, and can be had with lots of gearing choices and the G80 for locker, which works pretty well IMO. Just an idea.
Good Luck!
chris