06 f350 ring on front passenger side.. what is it? How do I reinstall it? And could it be the problem with wheel pulling?

 

Looks like a knuckle seal to me. So this is a 4WD truck? Check all front end components for play (use a long pry bar). Check the front axle u-joints for stiffness.

-- Dave

 

That’s just the dust seal for the axle tube, the real seal for the oil is inward near the differential. It’s really a non-issue and most trucks with a few years and miles on them have the same dust seal flopping around. Mine have been just like that for about 10 years now.