I did some more reading through here and saw some posts about NHT owners upgrading to 5100s. It seems that most people are pleased with the results. The ones who didn't appeared to have too much sag, stacking lift kits, etc. all things that probably created their own issues.
I also saw old screen shots from web sites saying the 5100's were for all except NHT. However on TireRack, Amazon, and even Bilstein's own website there is no mention of these not being applicable for NHT equipped trucks. Perhaps something changed in the shock, or Bilstein has done more testing, but it doesn't appear to be an issue any more.
I am going to go ahead and purchase them based on what I have been able to find. I also bought some Firestone Ride-Rite air springs to assist the load in the rear. I currently tow an 18' open car trailer, but it is all steel. That with the race car on it, spare tires, fuel is around 7000lbs. Then I put a bunch of tools, cooler, more fuel, etc in the bed which is probably close to another 1000lbs.
I had a 2000 3/4 ton Yukon before this and it handled the weight no problem, but rode rough and got terrible fuel mileage. It got to the point I didnt feel safe having the kids in it (old and wearing out) which is why I bought the '18 1500 NHT. Same tow rating as the Yukon but better ride and mileage. Th bouncy feeling does not give me confidence towing with any weight.