Some of those rigs have a combination of a short torque tube at the transmission, a carrier bearing, and an open driveline between the carrier and the rear end. The enclosed U joint at the front can be scarce and expensive to find. The open joints should be available at any local parts store with a gray-haired oldtimer who knows how to read a paper catalog and use a set of dial calipers. The HO 52 axle has two ratios that are commonly available, "slow" and "really slow"- - -4.56 and 5.14. The 4.10 gearset that was used in the mid-1970's 3/4 tons with an automatic transmission is extremely hard to find. Try looking for a GM 14 bolt corporate axle from the mid-1980's with a better selection of ratios. The ones from a small box truck with single rear wheels will be about the right width for the 3600. Good luck!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!