I heard a grinding noise, put the R350 up on jack stands and into drive, all 4 w

Sorry for just getting back but anyway it's in the air now on jack stands, all 4 wheels seem to be grinding and don't want to go. Gets much worse when pressing the gas pedal ( wheels go as normal when not giving it gas). I opened the transfer case and the chain seems pretty snug as it should be.

I am having a similar issue, but it is sporadic. I replaced the front drive shaft and checked the transfer case chain. It didn't have a lot of slack, so I left it on. What mine does is that, occasionally, when I start to drive from a stop, I hear a loud grinding sound, and the car doesn't move. It feels like the grinding binds and stops the vehicle from driving. But after a few moments, it will drive perfectly fine for days. I feel that if it were mechanical, like the bearings on the transfer case, it would be happening almost all the time. So, can it be the transmission ISM? Or does it have an electronic diff that switches the power from front to rear drive shaft? Maybe the electronic servo that controls the diff is going bad? Additionally, I have noticed that the right rear wheel sensor is malfunctioning. I will replace it today and see if that resolves the issue. Thanks for any help.