Your axles arent centered, most likely from a small lift. You can do three things:
1) new rims - most expensive
2) wheel spacers - less expensive
3) buy a rear trackbar bracket to go with your lift.
If you don't have a lift, I can't tell you why it isn't centered, there could be a number of reasons. I would persoanlly say go with a more narrow tire, because you are probably running wider than stock, or to just go with the spacers.