Hello,
I had the same question (except for a 2002 instead of a 2003) and this is the response that Reagan Performance sent me about the Borla system:
"Brett,
You are correct that there werent many changes from the 01' to 02'
models. All of the exhaust hangers are in the same place, but Nissan
used a longer muffler canister in 02' and 03'. I talked to a Nissan
Master Tech and he said it should fit just fine. The reason I listed
it as 00-01 is because that is how Borla lists it.
The Borla system is the best one made for the Xterra in my opinion.
You can check out the install at
www.reaganperformance.com and
click on Project Xterra.
I think the exhaust will fit fine on an 02'.
Please feel free to ask any additional questions, I am here to help.
Thanks!
-Charlie"
Hope this helps