When I first came to this board, I was thinking of building a Tracker with IRS.  Basically it would be used as a high speed desert runner, but would have 4wd when needed.  The transfer case would get 6to1 gears to help the 31-32 inch tires, and also give you the lower gearing when crawling is required.  Install an air locker in the rear, and you could have rear steering brakes like a sand rail when the rear diff is open.
I was thinking of welding a rear torsion housing from a VW bug to the rear of the vehicle, (actually a modified version without torsion bars, and Fox air shocks or coil overs would be installed).  This modified version would allow the driveshaft to connect to the rear diff.  It could easily get 12 inches of rear travel, and up to 18 using 930 CVs with 4x5 trailing arms. ÂÂ
Or, it could be converted to a driveshaft version, instead of the CVs, and get up to 22 inches of travel in the rear.  This would cost alot more $$, and it would be one of those modifications that get done later down the road. ÂÂ
The only reason I'm thinking of ditching the idea, is because I don't think you can get a lot of travel out of the Tracker IFS.  It would almost have to be a dual Arm set up (like a Toyota) to get 12 inches of usable 4x4 travel without the risk of breaking CVs.  If I want to keep up with the VW BAJA guys, I need at least 10 inches of front suspension travel, 12 inches would be preferred.  Otherwise, they will leave me in the dust, because the front IFS could not absorb the bumps without bottoming out (front engine = lots of weight).
The rear diff is another issue.  I was thinking of shorten rear Toyota truck diff, rear Toyota Supra diff, or Corvette rear diff.  Regardless, the rear diff has to be as narrow as possible to allow maximum travel and also match the gearing in the front diff. ÂÂ
I think matching the front Tracker IFS with a IRS rear diff would require the most research.  So far, I think I'm better off using a stock 4wd Toyota instead of the Tracker  (Long Travel Toyota IFS Kits are already available).  But I like the looks of a Tracker better, ÂÂ

 the Toyota truck frame would be heavier, and would have to be shorten to have a good break over angle if to be used with 31 inch tall tires. ÂÂ
The reason for IFS and IRS for me was to have a 4wd that could fly down the forest roads with the VW Baja guys, and have 4wd available for the local Zuk clubs. It might even ride like a caddy.

Just trying kick around some ideas, I will be watching this build!  Maybe I will have to do some more research and find a way to make this work.