wheel back spacing?

[DF350]

OK i have 35x12.50 and want to go to a 37x12.50 and i rub already i think i have 5.5inches of back spacing now but if i go to 4.5inches will i still be hitting my leafs? or should i do smaller back spacing 4.0,3.5 or what im in need of new tires and tired of rubbing too! i got a 02 with what i think is a 4 inch lift and i have plenty of clearance to the fenders but would like to know about the rubbing. any help would be great Thanks guys

Daniel