[Overdrive]

Whoa, that tire's wacky! Looks like it's got one of those tumors...something like that can occur if your shock is no good...the tire bounces up and down at a very high frequency because the shock isn't doing its job, tire heats up a lot and that tumor thing can happen (I really have no other term to describe it). Not guaranteed to be the problem, but something to check. Does the rear suspension have any odd movements when you take bumps on the road?

Glad you found that out before too long...it also looks like it was rubbing on something, too. Is that part with the tumor the inner or outer edge of the tire? If it's the inner, see if there's any signs of it rubbing against something in the wheel well area.