Unlike true 4x4 rigs, Sx4 can never attain full lock, even in AWD lock. There will always be some slip, hence overheating at times.
Another thing, I believe AWD coupler is separate from differential itself inside SX4 rear end housing. So there shouldn't be any oil splashing onto the coupler just air, hence the air temp sensor.
Whoever had opened rear diff, should confirm or deny my statement, as I've never taken diff apart.
If, however, coupler is splash cooled by oil it could be done via external pump, cooler and some plumbing to further cool the diff.

Cheers...