I have both the ones used by AndyH and the universals, both sold at camping world. I prefer the universals they didn't vibrate as much as the others did and didn't effect the alignment of the real mirrors. I pull a PU that's wider than the ST and taller than the bed with an AC in the middle of the roof.
There are telescopic ones for other vehicles that are great for towing but they aren't made for the Explorer, Ranger or ST but can be gotten for the Durango and Colorado. If enough ST owners ask they may start to make them for us. Here's their web site. http://www.ttt-mirror.com/
We put a set of these telescopics on my cousin's Dodge 2500 and their great. We were able to add side indicators when we installed them.
Happy towing.