I've got some serious rear wheel steering on my 03 S-Type V8. Checked all of the bushings at the control arms (upper & lower), tierods, shocks, s. bars, etc. and everything seems ok. The only other possibility is the sub-frame bushings which do appear to have some movement.
Has anyone ever changed these bushings and is it a really big job? Are they even available or do you have to get a whole new subframe (which is the case on some cars)?

 

JEPC should show you the answer to parts. Free download.