99 Durango
 

My Durango has a grinding noise in O/D if you turn off O/D the sound stops. Shifts fine just the noise in O/D. I'm thinking its the torque converter, because I believe this is a three speed with a lockup they're calling O/D?

No that is a true 4 speed.
It could be a 44RE or 46RE. It all depends on the engine you have. Someone will correct me if I am wrong on the numbering. Either way it is a 4 speed. The OD off is really putting you down into 3rd. I do not think that trans is capable of 3rd LU. The easiest way to find out is do 50 MPH switch OD off it should drop down into third. Then depress the throttle slowly. It should unlock and then down shift. That trans is not capable of a locked shift either.

durango
 

46 re depends on engine size, sounds like bad od section(planets)

durango trans
 

It has a V6. So the O/D is electrically controled similar to the 4l80E?

V6
 

If it is a V6 then it is a 42RE. That is just a three speed with a OD unit stuck on the back. I agree it sounds like a bad od unit.