I would suggest going to the tire store that you may purchase tires for 15 " rims and have them fit some rims to your Nitro. You are correct the same set-back for your original rims may not clear for the 15" rims, and may need to be increased or decreased to clear. Even if you have tires or rims already you should get the tire shop to recommend what to use, even if you don't plan to purchase. My experience has been they like something different to work on, instead of just replacing tires! If not, try another. Sorry we could not help, but it is not a regular request, but I'm sure someone has done what you want. They had a set of mounted 15 inch rims with tires in their garage, with the same bolt pattern, and just wanted to see if they would fit. Don't know the answer. Good Luck!
If you find what does work please report back to help other Nitro owners, in the future.
Nitro Year: 2007 (1 of 91,815 sold in 07)
Nitro Model: R/T 4X4 Stone White
CAT-BACK Exhaust, CAI, Projector Head Lamps
Fully-Equipped w/all factory options