Do you have access to machinist tools?
If you have access to either dial calipers or a telescoping guage and micrometer, you should check the bearing bore on the drive side to make sure it is round. Assuming there are no burrs, dirt, or other foreign objects clogging either splines on either mating surface, the only reasonable explanation I can think of is that the bore is out of round enough that the driven ring won't go back in.
In one of Chris King's manuals it mentions that some guys built their wheels with such high spoke tension that the hub flange is actually stetched so that the stock bearings actually fall out of the bore. To fix that particular situation, King stocks oversized bearings that they can fit into the enlarged bore. I've never head this before and this is just a guess on my part, but what if you have just a few spokes that are super tight on the drive side (and/or maybe a few that are really loose on the drive side) - maybe that makes the shell slightly eggy? While the guts are in, everything probably works OK. Pull them out, the hub is elongated slightly (we're talking 0.0002 - 0.0004" here - not detectable by the eye) and now you can't get the driven ring back in - and maybe not the drive side bearing either without beating it to death ?
I'd just perform a sanity check on your spoke tension and make sure you don't have any broken spokes, noodle loose spokes, or spokes so tight you can't even get any deflection out of them. Correcting the spoke tension may allow you to get the driven ring in. If nothing obvious jumps out at you as far as the spokes go, it's probably time to ship it back to King and let them look at it. They'll take care of it and get you rolling again for sure.
Let us know how this all winds up.