[HighPlainsCruiser]

To follow up on this I've done two reverse tests. The first backing up a very steep paved hill. Nothing out of the ordinary, RPMs stayed around 1K, no problems. I've also tried backing up a gravel road steeper than the parking lot where I had the problem, once again no problems, RPMs stayed at 1000-1100.
It's difficult to recreate the place where I had the problem, there was a gully which may have meant that either one wheel could have gone off the ground or perhaps the left rear and right front at the same time.
There was definitely a problem because no progress was made in reverse and it was necessary to turn around and proceed in Drive.
Still have yet to take it on a half way challenging off road trip, I'll try more tests when the opportunity occurs.