There are reports on this forum of dealers refusing warranty service to modified vehicles, and this includes wheels not purchased through them. I hope you have your original wheels - it would be simple to put those back on for warranty work, and the dealer would not have an excuse to avoid taking care of this.
It's not uncommon for a new car to need an alignment after a bit of use as things settle in. In addition, this problem can be exacerbated by
road crown.
I had this happen to me, where the alignment seemed fine when I first drove it off the dealer's lot but after some time I noticed the steering wheel was off. I simply took it back to my dealership, noted the problem, they checked and adjusted the alignment as best they could and ordered a replacement rear axle to take care of the rest - and it was all covered by warranty.
This is good advice if your dealer is giving you a run-around. Hopefully your dealer's service department doesn't give you a run-around. Until then, let's not panic just yet.
There are a couple threads that cover this subject. Follow the links for more reading:
Rear Wheel Misaligned?
Rear Alignment Problem FAQ
It has been a frustrating process for some owners. With a little knowledge, you can talk to the right folks at your dealer, Mitsubishi Motors North America, or it's Canadian representatives to get to the bottom of this.
We here are sensitive to the possibility that your rear axle is out of alignment but we really don't know if that's the cause without seeing the results of an alignment check. Hopefully, it's only your front end that needs a bit of tweaking.
Let us know how things turn out.