Well, here's the update ...
Received the used gearboxes yesterday ($290 for the pair on eBay), and got a little time tonight to install them.
Lookin' good! We are back in business. The replacement gearboxes are doing wonderfully; they were in perfect condition and actually seem a little "tighter" than the originals. Rocking forward/backward, I hear a little less gear lashing; they just seem like a little bit of an upgrade.
One thing I did notice is that I'm going to need elastomers at some point. The existing ones are still OK but not perfect - one in particular is starting to get slightly sticky and has a little bit of small shaved bits. But that's a project for another time. I'm sure I can do a teardown/gearbox replacement much quicker next time now that I've been through it.
Now, to analyze what happened .... I borrowed a buddy's torque wrench and mine is definitely defective. So I'm sure I overtorqued the bolts - I definitely muscled it trying to get the torque wrench to release. When I used my buddy's, it did so with very light pressure. I didn't realize how little pressure 22 ft-lbs takes.
So yup, I over torqued with my defective wrench.
Anyway - here's two photos of the left and right gearboxes. You can see one still has intact but compromised studs. The other one is the one that sheared off while testing the unit after replacing the tires last week.
Now, the bigger problem is that I can't just replace the studs without disassembling the entire gearbox. As you can see, there's not enough clearance to press the lugs out of the rear of the carrier plate - it'll hit the transmission housing. So even if I took it to a machine shop, there's nothing we can do without opening the gearbox, which is a task I really don't want to do.
