I saw it mentioned on another Board that Urethane swaybar bushings on the IRS make the bar VERY difficult to move by hand (a lot of torque needed to move the bar).

I bought and installed the Urethane (Prothane) bushings offered by MM some time ago. Looking at their site today, it looks like they are not the correct ones for 2003/2004 Cobras:

* These 25mm bushings fit the stock Ford IRS rear swaybar and its mounting brackets on the 1999-01 Cobra models.
Note: Does NOT fit 2003-04 Cobra models. Those years have a 26mm rear swaybar. Urethane bushings for those models are coming soon.

IRS urethane swaybar bushings, 1999-01 Cobra [6-1157-BL] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!

This explains why my bar is not prone to movement. Hopefully the new ones will be out soon enough, I ditched the stock rubber ones.