I'm one of the guys who had my HO52 (3/4 ton coil sprung axle under a '69 C20) rebuilt before I knew anything about these trucks. Short version: don't bother unless you have spare parts. Swap it out if it's not in good shape.
It
had 4.10 gears in a standard carrier. After sitting in a hay field for several years moisture invaded the housing. The carrier bearings pitted, and led to the slow destruction. One of the spider gears siezed, and welded itself to the cross pin. The rest tore the teeth off and kept turning.
The moaning and vibration was evident when I first bought the truck. I had Randy's Ring & Pinion (
http://www.ring-pinion.com) take a look at it for me. I forsaw difficulties with getting parts, so I had them swap out the 4.10s for 3.90s (that are still available) and a detroit locker. Here's what the damage was:
3.90 gears - $250.00
New Locker - $550.00
Complete bearing set - $500.00
Labor - $300
Total - $1600.00 + tax
You can still get 3.90 gears, but aparently you can't get new lockers for them, so unless the carrier is still in good shape I'd say its time to either look for a carrier in good shape or plan on swapping it out.
The whole HO52/HO72 debate is a common misconception which starts when someone with a flashlight and some cleaner starts looking for numbers on the axle. First, the numbers on the housing mean almost NOTHING. I haven't found any reliable sources that can decode the numbers. Second, you can find "HO72" stamped in parts in and out of the axle (pinion nut for example). This leads people to think they have a 1 ton axle under a 3/4 ton frame. Some of these parts are interchangeable! Most shops will absolutely SWEAR they're all HO72 axles, but the chassis manual speaks for itself. The presumed history behind these axles is that the housing made by the GM corporation, the carriers were made by Eaton, and the axles were made by Spicer. Therefore, the axle is nick-named an Eaton axle, but usually it's called a Corporate Axle, with 10 bolts on the cover and supplies an 8 lug axle. Two options were available once-upon-a-time for these axles: an eaton posi unit (which are still around but hard to find in good shape), and the ever-ghostly 3.73 gear set. I've seen a set of these, so I know they exist, but they're impossible to find. My only guess is that sometime in the '70s you could by an aftermarket set for this axle, and some people got them, OR it was a dealership thing that people opted to swap out their 4.10s for 3.73s before the truck left the dealership.
Finally, the only real liability of these axles is if dirt/moisture gets in the housing. That means good axle seals, and a good seal on the diff. cover. Gaskets for this axle is impossible to find, so most places just use some RTV or Silicone II Window/Door sealer to seal the cover (seriously).
I've been running mine for a few years now, and I love it. I've had 3 people request to buy it once I tear the truck apart. I've had it out of the truck once when I rebuilt the rear suspension, and it was a heavy bastard. I probably won't keep it when I restore the truck...