I haven't seen any MJ-specific rails but it's possible that a set of XJ rails could be modded to work
MJ's uniframe flares outwards (approx under the seat) where XJ frames don't, so the tie-in pieces back there will be shorter than on a XJ product...and possibly angled depending where it locates.
Mark Hinkley/ORGS Manufacturing is setting his MJ up with Super Nerfs. I'm guessing they'll be like the S Nerfs for the other Jeep vehicles, which are the best out there. I don't know if it's something he'll produce or not. You might have to sweet talk him. Check it out at www.orgsmfg.com Check out Project MJ.
If you are removing the bed or trimming sheetmetal in front of the r wheel...the rails could be a whole lot shorter than an XJ. The metal in that bed-front area isn't 'structural' like the pinch-seam under the rockers.
On my LWB, there was little to no misalignment between the bed and the cab. YMMV