There are several explanations available on the Internet to explain wheel size, off sets and spacing on the Internet to explain this issue in great detail. It is far too much to post here and explain properly. For a 33 11.50 to a 12.50, lift size and type are important unless you are running Baja fenders and the cab mount cut has been done.
For example, for both of my trucks, I am running stock/Baja wheels with a 1" spacer (26mm negative offset, 2" lift on my 05) with 34-10.50 and have no rubbing issues on both trucks, unless I jump it hard in the dunes. With a 12.50, you will need at least a 30mm negative offset to avoid rubbing the frame and you will still need to change the upper control arms, 35mm if you want to keep the stock arms. Also, a big issue is wheel configuration, as 16" is the smallest rim you can use and even then, the inside diameter of an aftermarket wheel is critical to clear the brakes.
Hope that helps.
