Steering Problems '01 F-350

[dcc01ford]

I have an 01 F-350 7.3 4x4, 124,000 miles, all stock. My problem is that when I am driving down the road the truck seems to wander all over, more noticable at higher speeds. I have replaced the upper and lower ball joints, LF wheel bearing, steering shock, shocks, all within the last year and all seem to be in good shape yet. The tie rods are tight. When I jack the truck up and lock the steering column and turn the tires I have a little play in the gear box, I know there is an adjustment screw which I have tried, but it only seems to make the steering wheel tighter, doesn't help the "wandering". Am I in need of a new gearbox or am I just overlooking something simple? Any help is appreciated. Thanks, Dan