So which one of these push the rim out and away from the truck? I've found two wheels with the same backspacing but a different offset.

 

 

Negative offset pushes the rims out. My rims have -12 offset and 4-1/2" Backspacing and they stick out about a 1/2" compared to stock.

 

The stock rims are +20 offset, so anything less then that is going to push the rims farther out.

so from going from a +20 to a -12 you are pushing the rim out 32mm

 

The wheel width needs to be taken into consideration, too. A wider wheel with stock offset will stick out more. Add offset to move the wheel in, subtract to move out.

 

^^True, i was using the stock 9in wheel width.

 

so i got a +18 offset with 20inch kmc wheels with 35inch nitto mudgrapplers, 6 inch bds lift. just wondering if the wheels and tires will work. they havent came in yet

 

I don't think they'll work. BDS requires 5" or less backspacing with their lift. Your wheels have 5.7" backspacing, and stock wheels are 5.75" backspacing. Hate to rain on your parade, but its BDS's fault, they designed the lift for wheels with less backspacing.