the pinbox flexing upwards is frame flex. The pinbox is welded to the frame, if it is moving the frame is flexing unless your welds are broken. that is what this measurement is for. you are just measuring in a different spot, so you get a different number. another possibility is your bolts are loose. there should be no movement from the pin box bolted to the frame.
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Jason & Wendy
2014 Ford F-350 Lariat 6.7L PSD
2015 Keystone Montana 3711FL
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