I have also read of Goodyear tires rivering, but on front tires, not on the rear as you discribe yours. That is weird that only one of the dual rear tires would be doing that. The only thing besides being a tire defect is that maybe you had been running the tire low for quite awhile.
I will do some research and see what I can come up with, and if I find out anything I will sure post it here.
http://www.kaltire.com/uploads/images/k ... tionsB.gif
This link will show you what rivering on a tire is and what might be the problems. Hope that helps!
RayChez