I've got the 700R4 in my car. Would anything be harmed if I popped the tranny into neutral while going down a hill and turned the engine off so I would be coasting?

 

i would say no, but you would not have power steering or brakes during you decent and why would you want to do this?

 

I'm still trying to track down the vibration in the car. Coasting in neutral would take the engine out of the loop because it wouldn't be running. Also it would take the tranny and possibly the driveshaft out of the equation because they would be driven by the rearend rather than driving it.

 

As a way of diagnosing things like a vibration, it's acceptable to do.

Even in neutral, internal parts in the tranny will still be turning because they're being driven by the driveshaft. Once you shut the engine off, you only eliminate the entire engine, flexplate and outer case of the torque converter as a source of vibration.

You could just as easily put the diff up on jack stands and do the same thing.

An engine vibration, unless it's serious, will happen at the same engine rpm regardless of vehicle speed. You can determine that with the tranny in park. A driveline vibration normally happens at the same speed regardless of engine rpm.