scathcart

Wouldn't do a thing without milling a groove into the contact surface.

In general, polyurethane is poorly suited as a bushing material. Once the lubricant works its way out and washes away, the bushings start to bind and cause the suspension to bind. This is the squeaking you here. Graphite-impregnation does not completely solve the binding problem, the problem is the material itself.
Bushing upgrades are generally better suited to be made out of a more self lubricating material, such as nylon or delrin, or machining metal bearings with zerk fittings for lubrication.
Polyurethane is cheap, tho