I am looking at putting a 305/55/R20 on a +15 offset rims. What do I need to do to accomplish this? wheel spacers? lift? any and all answers will help!

 

I have 285/55R20 Nitto Terra Grapplers on set of stock wheels with a +19.5 offset with some rubbing at full turn to the left. Other than that they look and feel great.

 

I had 305/55/20 Trail Grapplers on +18 rims with a level kit. They rubbed the control arm a little and needed 1/4 spacers, but this is because of the aggressive side lugs. If you are level you shouldn't have an issue because of the lower offset. Stock height you'll probably need spaces but it might jut make it without if you are lucky. The guy who bought my Trail Grapplers put them on stock wheels with no lift and only had to run 1/4 spacers as well.
Hope this helps some