Different Tire tread depths?

My first question is this: For some reason, the tires on the right side of my truck have worn down very quickly. After only half the warranty mileage, they have gone from 13/32" new to only 4/32". The tires on the left side have gone from 13/32" new to 7/32". I had the tires rotated once at the first scheduled interval but have not done it since. The truck is in good repair and is aligned very close to specs. What could possibly cause this?
My second question is this: I live in Canada, and I am not comfortable with having only 4/32" of tread depth left on the two tires which are now on the front of the truck. Winter will be here all too soon and I will be needing to use my 4-wheel drive. I want to buy two new tires to replace them, but if I do this, will it tear up my transfer case which I had rebuilt last winter? I have heard that it will. My tire guy tells me that as long as the tires with the same tread depth are on the same axle, it shouldn't be a problem. I don't believe that.
These are good Yokohama tires, and for two years they have been excellent! Now into year three, everything is seriously messed up. Please help me out here, I'm very confused. Thanks!