Need Advice on my towing set-up w/ weight dis bars

I towed a lot of happy miles with a 2007 2500HD Classic pulling a 6,000 pound trailer with no WD hitch. I have a 2013 now and use an Andersen no sway hitch to eliminate the little bit of bouncing/"porpoising" that I have without one. The newer hitches are rated much higher for use with or without a WD hitch. In addition to other comments, you should examine your receiver for possible failure. If it is beginning to come apart, you might feel what you are feeling.