You could probably use any antifreeze if you change it annually, like the schedule says. However, using a heavy duty diesel antifreeze is cheap insurance.
The Amsoil ATH meets the requirements of the original fluid, Permatran. So, of course, you can use it in the front diff. However, using gear lube is also specified, and since it is able to withstand higher pressures, it is a better choice.
If your worried about low temperature performance, use a synthetic gear oil. AMSOIL Synthetic Gear Lube SAE 80W-90, has a pour point of -42F. And Sever Gear goes to -50F
The gear oil is not as thick as you think. It is quite similar to the hydraulic fluid. I really don't think it is an issue. There should be enough residual oil on the gears to do the job, even if the fluid gets slow moving.