Well, file this whole debacle under "Things That Take Way Longer Than They Should."

bestgen - I wasn't worried about it falling apart but was worried about being able to put it back together properly since you confirmed that the pinion nut does have to be set just right to get the right pre-load on the gears. 

Since I don't have an in-lb torque wrench, have never set up a differential before and it was all installed and ready to go I decided to try a different tactic.

I discovered that not only were the existing holes too small, but the pattern was different and only two holes at a time would line up so I bought a 25/64 drill bit which is just a half-a-hair larger than the 10mm bolts, rotated the yoke 90 degrees and using it as a guide I drilled 4 new holes in that flange.

I wasn't sure I'd be able to do a good job of drilling nice, straight holes laying on my side under the truck with a hand drill but it didn't go too bad and the bolts fit nice and snug in their new holes.

After that I filled it with oil and took it for a spin around the block.  The new transmission doesn't shift as easily into 1st gear as the old one but otherwise feels okay and there are no strange noises, at least at speeds up to 30 MPH on flat roads.

I just want to throw out a HUGE Thank You! to the folks who've offered help and encouragement on this forum.  I really appreciate it, and this is definitely the best 'yota forum out there for dealing with friendly, helpful folks.

mudmaster - if you could let me know what year that diff came out of just so we can make a note of the difference in flange pattern & bolt sizes for the different years that would be cool.  And, thanks again for hooking me up!

Only one weird thing is happening right now - I'm not getting the 4WD indicator light on my dash any more.  I swapped the sensor from the old t-case into the new one but the light's not coming on for some reason.  I'll have to check my reverse lights to see if they're working or not, too.