tire ballooning in the rear, but not front...

i had been noticing that my rear tires balloon a lot and the front tires don't as much for some time. check everything out and looks good. finally, the front diff gave out. so i dropped in a new one, put her back together and now the front tires don't balloon at all and the rears go way out!
the fronts peel out in reverse. everything seems good. i'm gonna rotate the tires and see if the symptoms follow the tires or the location.
anyone run across this?
i notice the rear tires seem much more unbalanced too.

p.s. my setup...
emaxx 3905
1515/1Y
MMM
4s/6s
stock tranny
traxxas cvd's all around (and center)
FLM 3905 diff cup
100k rear, 120k front
axial terra-izers