While Pro Comp doesn't list specific wheel backspace fitments, we recommend using wheels with a maximum of a 5.50" backspace for these upper control arms. Now, I'm not all that familiar with the XTR 18" wheels, but I believe they have a 44mm offset, which gives them just a hair too much backspace. If you're going to jump up to 20" wheels in the future, you shouldn't have an issue as long as they're at least 20x9" with at most a 12mm offset, though an 01mm backspace would be safer and open up more tire options down the road.