Thought I'd pass this find on to everyone. I got one about 2 years ago like this for $120. I haven't seen one this low in over a year. It can be a little noise maker, but I would never go back to an open diff after this. My truck just seems more firm.
Lock Right Power-trax Inertia Locker Ford 8.8 rear end:eBay Motors (item 110405621197 end time Jun-28-09 19:50:48 PDT)
(Disclaimer: I have no connection to and no idea who the seller is)