In the Dmax/HD trucks, the OEM of the diff (Eaton) specifically has stated that any GL-5 will suffice, be it dino or syn. Obviously syns are going to take a bit of heat a bit better and last a bit longer. The AAM axle with the Eaton diffs have been known to not overheat even under towing conditions as long as the diff is kept at proper fill levels. I personally hammered my truck about as hard as I could this summer (see my UOA) and I used Synpower in the rear this year. No issues whatsoever.
Since the ST oil is GL-5 rated, I'd suspect it will do just fine.
I have posted some related links over at DP many, many times. Please review them when you get a chance. I have posted many links and quoted my sources, as always. One of which is a direct quote from an Eaton representative about GL-5 being the effitive good choice, regardless of brand/base stock.
BTW - nice to hear from you, Byron. Long time; too long.