Probably the same one that allowed 2wd Z71's in the first gen twins...
AAANNNDDD I just checked... You can still build 2wd Z71's... Sigh....
Why not let people buy 2wd ZR2's while you're at it?!
....
I miss when the Z71 decal actually meant something...
The thing is, since the Gen II Colorados arrived, there isn't much difference in the standard suspension and the Z71. The rear suspension is reportedly the same and the front Z71 gets slightly different shock and spring rate. Ride height is the same with comparable sized tires.
In 2010 on the Colorado Z71 (even in 2WD like I ordered) there was a marked difference. First, you got the Z71 completely different suspension with torsion bars in the front and a higher ride height, larger beefier drive shaft, and 265-70R17 Full Sized Truck Tires. So even if you did not get 4WD, it was worth getting a Z71 Crew Cab.
My 2010 is still in the family with son in law. . I sorely miss the better and wider leather seats, CD player, roof mounted dual front grab handles, and more.