for the sake of the next person wondering about this - here is the best answer i've gotten
The bigger differences in the frames are that the Reign X:
* is a bit slacker (by a degree or so)
* has burlier tubing (especially obvious around the headtube region)
* has a bit lower BB
* fits a reservoir shock - larger 'glory hole' ('08 reigns do that now as well)
* uses a slightly longer shock (7.875" x 2.25" on Reign X vs. the 7.875" x 2.0" on the reign).
You could run shock with 2.25 stroke on your reign to get an extra .75" of travel. I've done this and a lot of others have as well with no issues. I don't know if the rockers are actually different though.