I'm assuming you mean this one ( the ruger 556 comes in several different configurations)
budsgunshop.com/product_info.php/products_id/120317 />-------------
That one would not be legal in NJ.
as it has to many banned features.
"The following are prohibited:
✘Semi-automatic rifles that are capable of receiving a detachable magazine and has at least ​two ​of the following features: 1.Folding / telescoping stock 2.Pistol grip 3.Bayonet lug 4.Flash suppressor 5.Threaded barrel 6.Grenade launcher"
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This rifle is a semi-auto, capable of accepting detachable magazine, and it has a pistol grip, Threaded barrel, and a telescoping stock.
Plus its got 30rd magazine to boot (whic is also banned)
Full details here
budsgunshop.com/feeds/state_reg/new_jersey_restrictions.pdf />----------------

Warren, to be complaint in NJ, and AR needs to have a fixed stock, and a non threaded barrel, (you can have a threaded barrel if a muzzle brake is pinned & welded). NJ compliant model currently out of stock >>>> budsgunshop.com/product_info.php/products_id/347 NJ restrictions here >>> budsgunshop.com/feeds/state_reg/new_jersey_restrictions.pdf

Ruger does make a NJ compliant model.
This one features a 10rd magazine. A non threaded barrel, and a fixed stock.
But its currently out of stock at buds
budsgunshop.com/product_info.php/products_id/347 />---------------
It actually looks like that ruger is sold out pretty much everywhere.
Some other NJ legal ars available for backorder here
dark-storm.com/new-jersey/.
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Or you can look on (G)unBr0ker or armslist.com