Vibration at 1500 RPM in Park

[Cactus]

I've been chasing a vibration at 1500 rpm that occurs both in park and while driving (2003 sport, 3.7L). No codes. It runs fine at other rpms. I've looked at or replaced the obvious culprits, i.e. motor mounts, harmonic balancer, loose hoses, cooler lines, etc. I'm looking for other suspects...could it be the sacked out stock suspension resonating? (I have OME springs/shocks on the way.) What about coil packs - I've heard they can cause a misfire without throwing a code? Spark plugs have been changed. Or is it just normal behavior? Thanks!