Looks like there are two replacement choices for the rubber bushing on the axle end of the UCA.

I just noticed this on the Steeda site. It's not listed in the 05-06 Mustang Full Line page. I found it in the general Bushings section of the Suspension page in the store:

555-4104
"Spherical Bearing for 2005-06 Mustang. Replaces upper trailing arm axle bushing with a Teflon lined spherical bearing. Housing is made from Billet steel for ultimate strength. Hardcore product will generate more NVH."

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$84.95

And this January release from Spohn Performance

M5-600
"Rear end housing polyurethane bushing kit for rear upper torque control arm on the 2005+ Ford Mustang."

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$16.95