[fintubi]

OK, here's some material new information:

1. As expected, the loose axle nut wasn't the cause. The squeak still appears now that it's fixed. But I'm darn glad I found it!

2. I'm realizing that time at speed alone doesn't precipitate the problem. It appears *after* going from high- to low-speed driving. That got me thinking brakes.

3. More testing confirms the issue is worst when "floating" between accel and decel, persists during light braking, and goes away during heavier braking.

4. Most important observation (in retrospect, this is a great test): when the squeak is being really pronounced at ~30mph after a recent run at higher speed, lightly applying the handbrake (just dragging it a little) makes it go away. Every time. Completely reproducible.

So here's my hypothesis: A braking event that puts sufficient energy into the rear rotors (e.g. slowing from highway speeds to a stop) causes them to grow just enough that one of the parking-brake pads is dragging intermittently with rotor runout. That also accounts for the issue fixing itself with continued driving - if the driving patterns allow the rotor to cool back down.

I'd originally rigged the parking brake with a little bit of cable slack, because the return mechanism at the rear isn't super powerful. But I need to double-check that it's doing everything it can to release the brake - perhaps some lube on the pivots.

I'm fairly certain at this point that there's nothing wrong with the diff, rear wheel bearings, or front end based on test (4) above.

Bill