My 1999 ML320 SUV has a chirping squeak - sounds like failing U joints used to sound on older cars - when I put the car in nuetral and coast, it goes away, accelerating hard it goes away, accelerating very slowly it sounds awful

Any ideas what it may be (there's no vibration at high speeds and if it's still making noise, it cannot be heard for the road noise)

 

These cars are know for faulty center bearing bushings.... usually you will get some vibration when accelerating or when driving normal.

Just climb under the car and check the drive shalf for up and down play in the bearing.

Good Luck