Check out http://www.teamdfl.com/bmw/e28/bushings/polysubframebushings.html. My recommendation would be do not mess with the diff and subframe mounts. You can use urethane on the semi-trailing arms, but leavve the rest alone.

Neil Deshpande
http://www.neilwerke.com

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I'm gonna try filling the open cavities in some new sub-frame bushings and I'd like to use some soft durameter urethane to stiffen them up, while retaining some of the insulation provided by the rubber.