Steering Gear Box problems of the 1996 Mercury Grand Marquis

I believe that there is a serious problem with the steering gear box in the Mercury Grand Marquis 1996 and any other vehicles which use this same component in their vehicle. The problem is that there is excessive play inside the steering gear box. I am sure that this problem is the gear box not some other component of the system. The seriousness of the problem is that the play inside within the gear box is translated to the road in the form of poor handling,swaying etc. I changed my upper, lower ball joints,tie rods, pitman arm ,tires,sway bars,idle arm,and aligned and also replaced the gearbox with a rebuilt gearbox this completely different gearbox rebuilt mind you has the same problem as the previous gear box. This type of vehicle is driven by a lot of seniors, cab companies ,etc. The newer model vehicles of this type use a different gear box which I am pretty sure it does not have the same problem as the older models have. As far as having the old part I gave it to the company that I purchased the newly rebuilt one from as a core.