just wondering if someone could help me understand how you can put different size tires on the same rim and backspacing. I got a good idea but just looking to understand better. (ie if i was to change rim size then what would I be looking at) Thanks

 

search for back spacing / offset and read.
talk to a tire store and understand tire profiles
consider the width and height of the rubber

I replace a 10x18 inch wheel with 295/40 tires
with 20 inch wheels ( 6.25 ich back spacing) and tires
talk to your tire guys and be sure you know the wheel tire combination will fit when turning and hitting bumps etc.

Also if you lower the truck later will the size fit ok...

Good Luck