I am ordering a set of cragar black rims in 15x8 flavor for my 92 xj. Do I need center caps or are they just for looks? is there a difference between 4wd rims and 2wd rims? when I put the rims in my cart,it didn't specify whether the rims were for a 2wd or 4wd,but the bolt pattern was right. I also need to know the lug nut thread size because I will need to order new lug nuts. thanks,Ben