I'm not so sure about that...
"Original rear end may be replaced with another from the same
automobile manufacturer; truck rear end prohibited. Aluminum
center section permitted only on vehicles that were originally
equipped with same. Distance between backing plates may not
be changed. Any gear ratio that fits third-member case or
housing permitted. Limited-slip or ratchet-type rear ends
permitted. Reinforcement of spring perch permitted. Spools
permitted only with aftermarket axles. Aftermarket axles not
required for front-wheel-drive vehicles. Larger brakes may be
used. Replacement rear end may be narrowed or widened to
obtain original rear-end width. Bracing or beefing up the rearend
housing permitted.
Swing axle differential may be replaced with conventional
housing; stock trailing arms must be retained, may be beefed up
and adapted to housing, must retain transverse spring. Must
install Panhard bar. Frame may be notched for driveshaft
clearance. Distance between OEM backing plates and OEM
wheelbase must be maintained. Coil-over shock or four-link
adaptations prohibited. See General Regulations 2:11."
As I see it, GM's '(9")" is now in the mix. We already run aftermarket 12-bolts...what is the difference with this one? "Original rear end may be replaced with another from the same
automobile manufacturer"

Let the legal interpretation of the rules begin.
