go to a ford dealership and ask for a bottle fo friction modifier. Put that in with the rear end grease. Give it about 500 miles to work in. That should fix it. I have an auburn unit in my rear end with 373 gears. Worked for me. I will admit i will occasionally get a slight rub everynow and again. But not often.
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88 LX 5.0 Ragtop
"Desire"
14.29 @ 96.45 mph
Best 60ft 2.19 YUK!!!!
Best Mph 98.9