My son was driving around 45 mph in his 2001, 1500 4x4 he heard a noise then the truck stopped moving. I removed the pan and observed clean fluid with little shavings on the magnet and no apparent broken parts of plates or clutches. When the engine is running I hear a whining noise, almost like a power steering pump makes, but the truck will not move. I checked the transfer case to see if the chain was broken however, it seems to be fine. Could it be the torque converter? Or is it more likely the transmission? My son is on a tight budget and can’t afford to experiment by randomly changing parts till we find it. I need to be accurate in diagnostics and fix it as inexpensively as possible. I was a diesel mechanic in my former life and retired from it 15 years ago and things have changed a lot since then and having never been real strong in transmission diagnostics anyway, I can use all the help I can get so any advise would be greatly appreciated.