After a couple of attempts, I was able to speak to someone at Sharp who was knowledgeable enough to point me to their Canadian website where they have downloadable spec and owners manuals for the 642.
Look under the "support and downloads" tab here:
If you compare the specs for the 642 against the 632 (still listed on the Sharp.ca site) you will note they are both listed a back lit displays but differ in display technology. The 642 is listed as just a UV2A panel and the 632 is listed as an X-Gen/Ultra brilliant display. This probably accounts for the difference in contrast ratio but may not matter to most end users.
In hopes of gathering more info I posted to a couple of the other Sharp 80" forums but only managed to stir up debate on whether this is really a back lit set or edge lit. As I mentioned the Sharp info states back lit as does the Costco US site but the Costco CA site lists it as an edge lit set.
At this point I have to believe that the Sharp info is correct but they have been known to make mistakes. I do know from seeing a 642 in person that it shares the same chassis, inputs, etc. with the no longer available 632 which leads me to believe it is really a back lit unit.
I am sure that Sharp cut corners to meet a sub $3k price point - this would explain the difference in display tech - but to me this could be the little brother to the 632 and a good value given the similarities.