So in fact it wasn't a tensioner problem. The harmonic balancer and drive pulley had fallen off. The rubber part was completely sheared.
But somehow the belt had remained in place.
I also discovered that the reason I couldn't move the tensioner was that I was moving it in the wrong direction.
Some comments from the mechanics indicate that this is known problem for late model Rangers, and Ford is completely out of stock of spares.
So going aftermarket.
Could have been worse I suppose.