No reason why you can't replace just one, or two, depending on wear. Road tires barely start out with maybe 7-12 32nds of tread, so worn tires aren't much smaller than brand new ones. It would be ideal to replace both tires that will be on the same axle, but I'd say it's far from necessary. Just get the same size, and same brand/model tire, if possible, and don't worry about it. The differences are going to be very slight. The only thing you HAVE to do is have the same size tires on each axle. Some AWD cars have staggered wheels/tires (bigger on back than front), if I remember correctly, and they're made to work like that. I could be wrong about using 4x4 with one size in the front, and others on the back, but I don't see why that would be a problem either.
If you had 3 worn the hell out mud tires, and one brand new one, that could potentially be an issue.
Last edited by KMAC0694; 06-11-2014 at 02:28 PM.