I centered the center bones on the "X" axis, deleted the left half of the body, centered the center spline, CFA'd the right half of the body and ran the MirrorBones plugin with the tolerances set to .3 (I think it would have worked set lower). It appeared to work fine. I didn't go through and check any of the rest of the rig, so I don't know if everything is where it is supposed to be, but, I think I can safely say that the problem is probably either or both of the things I changed. I didn't fix the groups, so that needs to be done.
Hope that helps, Paul.
PappaBearRigged_V3t11_updated.mdl