Even with all that explanation, I really think it's a balancing issue... or possibly even a bent rim (although I'd think that to be somewhat unlikely). Try switching wheels, front to back, back to front, just to see if the vibration stays at that back wheel. If this is the case, I'd probably take the caliper back off, and clean everything well. I'm not sure if there are bearings in the rear hub assembly (i'd like to assume so... but we all know where that can take us), and if a bearing is going bad and allows a bit of a wobble, it's another possibility.
Hope something I said helps ya out
~Eric
