Bigger tires mean changing Axle?

I have been reading post about tires, and going to bigger ones...we have a 1998 2518 that we just bought (in Sante Fe, NM and drove back to Minneapolis, MN) along the way one of the tires developed a bulge. All three tires, the spare and the two that are on it are different tires, but all are 13". When we called TM they said in order to go to bigger tires, we would need to purchase a new axle that would have the bigger tires on it...What say you? Help please. Can we put 14" on the current axle?

Harry and Kim