When backing up in my 1997 F250 diesel, the front tires lean in at the top so badly they rub and make reversing difficult.

wheel bearing failure allows wheels to lean it that top
next is ball joint failure followed by suspension bush failure
Take it to an accredited suspension shop and when finished with the repairs have a wheel alignment done

Wheel alignment is out of specs. Loose front end parts

If all the tires and rims are the same size I doubt only one would rub. I would lean more towards a bearing problem or half shaft problem

Front CV joint noise.

If its not the bearing and your Mercury is front wheel drive, it could be the "CV" joints.
It could be both....

Is the wheel rubbing the brake caliper?

Is something like the wheel well liner rubbing the tire(airflow pushing it into tire)?

Jack the tire off the ground and rotate it by hand. Push in & out at the top, sides, and bottom of the tire. Little or no play is considered acceptable. If you get a "clunking" sound and/or a lot of play the bearing(s) are bad. (If front wheel drive, chock rear tires, put transmission in neutral so you can rotate tire.)

Hope this helps.

It sounds like you may have a wheel hub bearing going bad . Jack up front of truck and spin each tire and listen for any noise it should be quite and spinfree if you find it's bad you must replace the whole hub you can't just repair the bearing this is a common problem on the f250

this could be a bad inner axle joint check to see if the axle boot on the left side is torn or missing also check the wheel bearings and tie rod ends by jacking up the front of the vehicle one corner at a time and grab the tire at the 9 o'clock and 3 o'clock position and wiggle it back and forth looking for excessive movement in the tie rod ends then reverse your hands to the top and bottom of the tire and try moving it from top to bottom checking for excessive wheel bearing movement it shouldn't move in or out at the top or bottom

8 or 4 cylinder? 4 ain't that bad about half an hour or so. the 6 is about an hour or more depending on your skill and tools.

How tight does the brake line get when turning? How is the sturts, does the tire rub when turning? Did you replace the inner and outer bearings?

You should have the twin arm suspension. there's just a few things to check. inspect the pivot bushings on your lower torsion rods(from the axle back to the frame), check the upper and lower ball-joints. and last check the wheel bearings.