Its hard to say you can't jack the back half up and test these bikes aren't built to very tight tolerances and besides your not going to replicate the same force you would under a load. if you shift early you can get some clunking coming from the crankcase area. I know you checked for play but have you serviced the differential?
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2008 V-Star Silverado 1100. K&N filter, airbox mod passenger floor boards, all light visors, ISO grips, Cobra slash cut mufflers, Saddleman touring seat/ tourpack. Lots of little extras.