Trailer Tires?

I have a 2005 Boatmate trailer with 205/75 R14 Goodyear Marathon Radial tires. This weekend, while wiping the boat down, I noticed an odd spot on the tread of the tire. When I got home I jacked the tire up and spun it on the hub. The tire was way out of round and bubbled in the center. I guess the belt on the inside separated and caused the tire to become out of round. My question is, should I replace all four tires or just the bad one? The others still have plenty of tread. I have been reading some horror stories about the tires online and am thinking about switching brands. Anyone else have troubles with Goodyear Marathon? What are other options?