FWIW, I just replaced all four the other day and even though I wanted to only replace 2, I wound up replacing all four anyways, for two reasons. First is the TOD issue - yes, you want your best tires up front, but if the difference in tread depth is signifigant (more than 3/32) it can mess up your transfer case (according to the folks at Discount Tire, anyways). Of course, if your TOD rarely ever kicks in, you'll probably be just fine. The other reason is because it'll look better. I went in to replace only the front. Nitto Terra Grappler AT's have a tread depth of 15/32, and since my rears still had 7/32, I figured they were only halfway worn. But once I saw what they looked like compared to the two new ones up front, I had to get them replaced. Just something to keep in mind.