Ok I know that I have to have certain backspace wheels to make a 3" work. But what else is entailed in a 3" vs 2"? I have a set of american racing wheels off a 91 Ford truck but I am not sure how to measure the backspace on the rim. any help is awesome.

Check the center hole first, if it's
not 4-1/4" they will not fit over
the front hubs.

After that backspace is mounting
surface to edge of rim on the back
side of the wheel, 2-1/2 to 3" should
be a good backspace

Wild

Ok I know that I have to have certain backspace wheels to make a 3" work. But what else is entailed in a 3" vs 2"? I have a set of american racing wheels off a 91 Ford truck but I am not sure how to measure the backspace on the rim. any help is awesome.

I don't think you have to have anything 'special' to make a suspension lift work, maybe to get larger tires to fit you might want different rims.  

But a 2 inch vs 3 inch suspension lift has nothing to do with tires as far as I know.

~Nate