I'm thinking the problem lies in the way my tires are worn.
The back tires were up front for a long time and I need front A-arm bushing badly. 
The front tires tend to get worn on the outside first while the inside of the tire is still practically brand new.  (worn at a side to side angle)
And with the tires so big (255/60's) thats a lot of width to be angled like that.  so Im thinking since the tires arent "flat" across the bottom, they end up basically being on one edge and not necessarily flat on the ground when mounted on the rear.  I think I can just feel this unevenness more when im crawling at idle speed
does this make sense?

It could be...there would have to be highs and lows in the tire not just flat across..usually cars that are really in need of an alignment will wear the tires like you describe.. but to really feel it they should have a wave appearance to the tread..not just worn on the outside and tons of tread on the inside..meaning the tread is uneven all over the tread pattern..if you run your hand over the tire does it feel like a wave or is it smooth tread all the way across..kinda hard to explain..maybe a pic of the tire from the side would help.. Rich

no, tire is smooth all across. Just a lot more tread on the inside than the outside.
I was thinking that when I moved them to the rear, only the "high" side is essentially touching the ground until it gets worn. im exaggerating this a bit, but each tire doesn't sit as flat (side to side) in the rear as it did in the front now

Makes sense IF the rear alignment is off a little. The tire may be trying to slide off in different directions due to a toe in-out problem, and the sorta snapping back....that could give you the sensation you feel, since the tires are worn like you describe...
Kinda like a shopping cart with a screwed up wheel....I know ya'll know what I mean...

I HATE gettin' carts like that!!!!!