I think the only real way to figure out what is going on is to weigh your truck, your combination W/O WD connected and then weigh it again with the WD hooked up. That way you can determine the actual tongue weight and how much weight is being transferred back to the front axle.
This might be the first story of issues with the Hensley/Propride hitch I have ever read. From reading the Airstream forum you would think that it is impossible for this to happen because these hitches are designed "to prevent sway" not just to dampen it down. I do believe this claim but like all hitches, you will have to get it set up properly.
You can call Shawn at Propride and he will be able to instruct you as to how the set your hitch up. I have heard lots of good feedback about their customer service.
Driving 55 is safe but my sweet spot is right at 65. No sway at all with my Equalizer which is a far inferior hitch when listening to Hensley/Propride owners.