[Aussie Mike]

If you have a think about the design of the AU rear you will realise that these nuts don't do much more than stop the axle from coming loose. The AU is a double wishbone set up and there are no suspension loads on the axles. The axles use CV joints at each end instead of unis ad CV's wouldn't take the longitudinal load any way. The rear hub carrier with it's bearing installed is designed to support the wheel without the axle.

The hub uses a double tapered roller bearing and it's retained in it's housing by a large circlip. The hub presses into the center of this bearing and the axle spline runs through from the other side and keeps the hub snugged into the bearing. The spline is a tight fit into the hub and you need to use the nut to pull the axle spline into the hub spline. I think what you might find is happening is that the bearing may not be seated completely into the carrier or something is binding. The load on the nut won't affect the preload on the bearing because that's fixed by the thickness of the 2 halves of the outer bearing race. The inner race is 1 piece.

If you've seen the piss weak stack of 3 thin stamped steel nuts they use you would agree that they can't be holding much.
I think the amount of heat that will get to the nylock will be negligable but checking them will be part of my regular maintenece schedule just like checking wheel bearings and brake pads. By all means use the factory set up as that's what it was designed to use. I just wasn't very impressed with it and feel safer with a big solid nylock on there.

The thread is a 24mm or 25mm metric fine from memory. The bolt place I got mine from had to order them in.

Cheers