Ripper,
Let me see if I understand correctly. Do you now have Sumitomos on all four corners or just in the rear? What size are your rear tires? Are they larger than the fronts?
Sometimes if you put a large tire on the stock rims the tire can cause the sidewall to walk some. I put 275ZR40 17s on the back of my stock 2000 rims and they walk a little, but not much. It almost sounds like you have a toe out condition. Perhaps you should try taking it to another Ford dealer.
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2000 GT --
Black, 5sp, Mach460, Bassani CatBack & X-Pipe, MAC Cold Air, 70mm Tbody, Hurst Shifter
1966 Mustang Coupe --
Tahoe Turquoise, Pony Int, A-Code, C4 Auto
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scott@pearland.com