33's came stock from the factory? You sure??? I know they came with 235's stock but I seriously dont believe they came with 33's.
You can run 33's stock, hell for that matter, with a little fender trimming, you can run 38's without a lift. But the downside to that is you will almost need rear steering because you wont have much if any turning radius. I prefer 33's or smaller on the 78-9 broncos because they have a very small turning radius. My 78 will turn around in almost any area with the 31's that I have on it.
I bought mine with 31's on it and quickly sold them. I put on a new set of BFG A/T's 35X12.50-15. I had to do minor fender trimming but it was worth it. My 38's on Hummer rims will be on soon with a little more fender trimming, if required. Still, I have no lift. I'm going to make new radius arms with a little 'kink' in them for turning clearance, though.