You used an impact wrench to what setting? They only call for 100ft/lbs.

Also, are you sure you used a wrench to secure both sides? This sounds like you may have cranked the gun and the on the other side just spun.

I just installed Bilsteins and I noticed that with the nut/bolt tightened down lightly it would do as you described (lateral wiggle) which caused me to worry. Then I torqued them down to spec using a torque wrench and sure enough no more play. Just a thought...worth double checking.