Just replaced ball joints tie rods stabliblier front bushing on my 04 super duty 250 now it rides great not thunking or anything but when you get going 45 -50 miles per hour it takes off to the left you must slow down to get it back under control could the steering box be going out or or the power steering pump going bad.
A bad power steering pump won't cause the problem you describe.
A bad steering box will generally just get loose & sloppy causing the truck to wander from side to side and make it hard to keep it between the lines.
When you say it takes off to the left, is it just drifting to the left and you can steer back to the right or something more serious?
If you have a 4x4, there is a track bar that goes from the frame under the steering pump to the front axle. It has rubber inserts in the ends which have been known to get worn and get sloppy.
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