Hey Nix.
How much restriction/binding are we talking here?
If it's just a little, it might simply be your brake pads getting reacquanted with your rotor. Have you take the bike for a brief spin? If so, was the rear brake rotor hot?
Check the exploded parts diagram on
http://www.buykawasaki.com/ and if you're sure there are no extra parts or spacers and that you've torqued the axle nut to the correct torque, it should be good.
The worst-case scenario I can envision is your tire-installer popping off your sprocket/cush-drive hub, the little spacer falling off, and the hub being reinstalled back on the wheel without that little internal spacer. Not a major deal, but you want to get that spacer back in there before riding.
Good luck and keep us abreast (or two) of the situation.