2002 GMC sierra

Hey everyone so this past weekend I took my 2002 GMC sierra 1500 to the snow. On my way home (about 4 hrs) i noticed that it felt like the rear end of my truck would drift and I came to and stop and then made a right turn. The truck has a 6 inch lift and 35 inch mud terrains. I have read some forums that say its a bad wheel bearing but have not read anything about the possibility of posi trac getting stuck on. Is there any other possibilities that could cause this slow rear end wander in my truck? Should I check the oil in the diff? any other suggestions would be great.