did some looking and you may want to go this route.
http://www.nissanpartsconnection.com
They have the front struts for $116 a piece so that puts you at about $400 to replace everything if you install it yourself. SO either way, looks like there is no easy way out, I would ask the stealership what specifically caused the shocks to wear out, they can't just say it was the wheels without explaining how the wheels played a part in wearing the shocks out.
Sometimes they are bad about blaming problems on anything aftermarket that you put on your car. I can see that if the off-set was WAY off that your wheels could be rubbing the shocks themselves, but i've never heard of just the wheels contributing to worn shocks and struts. I mean my local nissan dealer puts aftermarket wheels on about half the cars/trucks they sell. You're not on eibachs though right?