I spotted this on Ebay. Not a bad price. It would allow you to keep your stock trans and just shorten the rear driveshaft. No overdrive allowed while 4X4 is engaged though.
http://cgi.ebay.com/ebaymotors/Gear-Vendors-Overdrive-4x4-1345-T-18-T19-Complete_W0QQitemZ300271232984QQihZ020QQcategoryZ46104QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
