[Christ]

Drilling a simple hole in the diff cover would allow a thermal sensor (like a water temp gauge sending unit) to be bolted into the rear diff and sealed. It would have to be in the bottom, where the oil is likely to pool around it.

To help keep temps down, change the fluids out for lighter synthetics, unless you're already using SynchroMesh in the tranny. If you are, LEAVE IT IN THERE.

Rear diff can use something lighter than the GL90 that's likely to be in there.

You won't lose much by opening up the gearbox and diff housing to get some air, but you'll lose alot if they need to be cooled and don't get the air they need. If you're unsure, open them back up partially, and add some heat sink-like structures to them. Even epoxying some simple aluminum fins to your diff housing and the bottom of your gearbox can seriously affect lube temps.

You could also leave them closed up and (if you're feeling really frisky) duct your cabin air out through that area, instead of where it normally goes. The drawback, obviously, is that you must now always have the vents or windows open.