I researched the source of a 500 Hz vibration at idle, mostly in the
cab and I think I've located it as being the exhaust resonator
touching the frame.

The questions are:
1.    Will replacing the struts increase the clearance between the
resonator and frame.
2.    Do the helical springs loose their springiness and collapse, making
the repair above ineffective.
3.    How do I tell if struts are "leaking" – everything looks pretty
oily

Notes:
1.    I replaced the front motor mount which was chewed up.
2.    The other two motor mounts appear intact.
3.    Engine size = 2.6L

Thanks for your help.