I think your problem is the lower strut rod bolt and its fit through the welded timbersled backbone. The bolt is a slightly loose fit to the holes, i.e. 3/8 bolt, holes are slightly bigger. any small slop between the bolt and the lower strut mounting holes will add up to a huge amount of movement at the back bumper. I always snug the bolt and then do a final tighten once the bike is weighted sitting on the floor. I think I remember they even suggested this in the instructions. I'm still running a 2014 kit and mine has gotten really sloppy over the years. its kinda sucky but not really that big a deal. just don't let the bolt get loose on ya or those holes will get pounded out faster and bigger.