What backspace is good to run 33*12.5*15, with a 4.5 in lift? ???

 

3.5-3.75 would be best

 

And that will keep them from rubbing the LCA and fender when flexing

 

With 33s you will probably hit your fenders when flexing, but you shouldn't get any rubbing on the LCAs. Did you remove your fender flares? What lift kit do you have? I know some manufactures list the best BS on their website.

 

I am going to trim them shortly...I have a zone 4.5 ...they say 32's with a 4.5 bs....but I want to go the next size up on tires