I have a 1999 f350 7.3 diesel single rear wheel long box. I have a problem with the steering being extremly loose on the highway especially. This is amplified when I haul my 25 foot travel trailer. To date I have replaced steering boxes, replaced upper and lower ball joints, replaced steering bearings all to no avail. Is there any fix out there that will rectify this problem. It is a real battle to keep it between the lines and you can not take your eyes off of the road for a second! Any ideas would be appreciated!

Thanks
Gene