So, I've finally got all my idle problems fixed: New air filter, cleaned ICV and intake, replaced intake manifold gasket, replaced Crankshaft Position Sensor, replaced O2 Sensors, replaced spark plugs.

My car idles really smooth when the AC is on, but when I turn it off, it's still kinda lumpy. I know it's to be expected of a V8, but is there anything else I can do to fine tune it?

It's like the vibration of the AC compressor is cancelling out the vibration of the car as it runs w/o the AC on...

You sure it's not something like engine mounts?

No I'm not. That 's what I thought the idle problem was originally but in turned out to be largely caused by a vacuum leak (as well as the other stuff mentioned)

How can I check to see if the motor mounts are failing?

Ask them.

A visual inspection is the best way.
Or if you're man enough, give the engine a few sturdy tugs.

Failed engine mounts will most noticeably reveal themselves when you're on/off the gas in first gear. The engine will shift back and forth, and there's no doubt about it.

How many miles?