Most places that require a roll bar on a Blazer when the top is off are referring to only the rear section, since there is no longer anything over the passenger compartment. The front already has a 'cage' built into the body. The smittybuilt roll bar for the rear is an add on section to a front cage. I believe this is the way it goes with Smittybuilt. You may be able to adapt a rear section to work stand alone, or else you would need to get the front hoop also, if you were not already planning on it. Just something to keep in mind. I would recommend doing the whole shot, front and rear. For safety and also it will brace the flimsy Blazer body quite a bit and stop the flexing of it.
The smittybuilt cage(how ever you do it_ should work for you to pass the "needing a cage" to wheel requirement. We have the same rules for the sand dunes around here.
Just another note... its not cheap. I looked into around here and not many people carried it, and the few that did wanted like $800 for the complete cage.
Ken H.