If I'm not mistaken, and I may be, and we all know I'll be corrected if I am and that's a OK, but radials you replace in pairs. So if you have one that is excessively worn, find out why (alignment usually), fix the problem and replace both on that end of the truck (both fronts or both backs), cause it will throw wear/alignment off if you do just one.

That being said, if you replace two, do them on the front. The fronts wear faster due to more weight on the front (hence regular tire rotations) and you can afford to lose traction in the back better than the front.

Just my .02...

-G