Hey mate.
Went to hotham twice in the last few weeks. First time the road was covered with snow and icy sections, so i used high 4. Disengaged it when on the non slippery black stuff though.
Weekend just gone i only used 2wd, and thats when parking on the snow also. Had no problems. We had a dump during one night also.
Hotham is not a steep drive however, never been to thredbo so not sure how steep it is. I wouldnt worry about taking the muddies off though.
I have Maxxis Bighorns also. Just drive to the conditions. Snow on the road you still get plenty of traction, its the black ice that sucks ass.
Oh, and fill up with Alpine diesel.
Pete