4 years old? If they are 4 years old they must've been sitting for a long while, in which case they may have flat spotted. What brought you to the conclusion that they are oval? Rough ride? How often did you rotate them? What kind of air press. are you running? And when you say you a lot of miles on em, how much is alot?

Someone might want to correct me if I'm wrong, but as far as I know, tires can be oval as a defect, but good tires do not just "become" oval.

I have had very good experience with 3 sets of SSRs, 1 set of the 31X12.50X15 and I am currently on my second set of 35X14.5X15 and they are great. They are quiet, easy to balance, ride smooth and I got 43k miles out of my first set.