The IRS axle is defently the "hook up" as i am sure DavidL would tell you, lol . I have been using their 12 ball yellow axle diff for the past 6 months or so, and i loved it, i literally had to rebuild it only 1/5 the amount of times that i had to rebuild my 6 ball diffs. i just picked up the new graphite axle w/ blue anno'd aluminuim on it for the nats next week (which is D ringed instead of pinned like the yellow axle, so its easier but works just as well). I know that Scotty Earnst used one at the birds and got like 5th or something in stock 12th, he also used one at the carpet nats, but was having some really really bad glitching problems and ended up running mo denton's rev. 3 for his last qual and his main. however the chassis was not causing the glitching.