dodge durango idle bad knocking noise

My daughter drove her Durango 70 in 2nd and it over heated. She noticed it when it was real hot just not pegged. she stopped and called She let it cool down and drove it a little but it was still hot so she had it towed.

I went to look at it and check it out and seamed okay went to take it for a test drive and got about a block away and the motor started making a bad knocking noise. I took the car back to the house and parked it. I trailered the Durango and brought it to my house.. Thinking it was a bad motor knock we got a used motor (For Free) out of another Durango I went to start the Durango yesterday to move it, it still has the old motor in it and the knock wasn't there, or just barely could hear it. The motor would not stay running at idle. The oil does not appear to have water in it either. Does this sound like we should still go through the motor swap or could there be something else we should be looking at?

Shawn