Rough idling

[AleksACK]

Hello everybody!
Need some help from professionals…
I have 00’ ML430, with 100k and some troubles.
The whole truck is really vibrating (almost shaking) while idling at 600 RPM. It idles OK at 3000 RPM but “backfires” at/above 4000 RPM – scary picture, looks the like engine is going to break off the mounts and fly away. Feels like it missfires – but I think it doesn’t because CEL is off. I was trying to keep it at 4000 RPM for some time to get a code, but, like I said it’s scary and I don’t want to make any other damage – no code.
I’ve replaced mass airflow meter a year ago (check engine light was on), also crank position sensor was replaced 6 month ago because intermittent no-start. Spark plugs were replaced a week ago, spark plug wires tested and cleaned – no result.
I don't know when this problem occured, as this is my wife's vehicle, I would guess 3 or 4 month ago.
Don’t know where else to look, please help.