Two kits available:
D.D. Machine "Box 4 Rocks" kit,
and the "Stubler"
The STubler is complete and just needs to be assembled, the Box 4 Rocks kit is just the plates to assemble the cases together, and a one-piece chromoly coupling shaft, you supply the t-case and cut it to fit (a small bit of welding on the aluminum case is needed, although even JB Weld can work if you do it right). Being the D.D. Machine kit is incomplete, it's also half the cost as well.
If you're battling an extremely tight situation with the fuel tank (some Rangers and Explorer Sport), the Stubler is about ¾" shorter, could be just enough to make it fit.
The D.D. Machine kit IMO I think is stronger, due to the one-piece shaft it comes with (stubler uses a two-piece shaft). I have yet to see a shaft failure on either setup however (don't ever use the box in low, with the rear case in HI, and you shouldn't have a problem).
Stubler kit:
http://www.4-x-fab.com/Stubler_info.php
D.D. Machine is a bit behind the times though (no website that I know of).
I thought I had a phone # for him, although I can't seem to find it now (A search here or on Google may turn it up though, it's been posted on the forum before, and on other forums as well (he makes Jeep 231 kits too).
Hope that helps