I have a '99 F-250SD that I swapped in the axles/steering/suspension from an '06. On my '99 rear axle both shocks are behind the axle no factory torsion bar, in newer generation Ford moved the rear axle passenger side shock from behind the axle to in front of the axle and there is a torsion bar. Seems straightforward enough moving the shock gives better control of wheel hop on acceleration. The torsion/anti-sway bar has the better on-road characteristics. I drove the truck for a bit (~2000 miles) with the axle and torsion bar moved over to the '99, but could only mount the driver side shock as there was a mismatch for mounts on the passenger side. I finally got the passenger side shock added (took a while to get a mount)...the first drive I go on and the rear axle went into an oscillation that all I can say is I am glad I was only doing 30MPH...it was tough to control that slowly. Any thoughts from a suspension/axle guru? What has happened makes no sense to me.