LOL....looks like you got the weight cops bent out of shape with the 8.1 2500 truck pulling it fine which it probably did in the previous owners mind. Some folks thinks towing is all about power which the 8.1 has hands down for that much weight.
The 8.1 comes in many MDT size vehicles. However the 2500 truck with the 8.1/Allison/AAM 11.5" rear axle which the one ton DRW had, doesn't have enough springs/rear suspension for a safe tow.
Anyhow a mute point.
A medium duty truck isn't needed for a 16k-18k trailer. .
You have enough truck but that much weight with a 1996 7.3 simply doesn't have the power/towing performance in the hills as you found out.
The newer Fords/GM and Ram 350/3500 DRW trucks have tow ratings up to 30000 lbs with 350-400 hp and more importantly 650-850 ft lbs of torque.
All three trucks have 9350 to 9750 RAWR which has plenty of load carrying capacity for that size trailer and even heavier.
I'm not a brand cheerleader. Simply choose the brand that suits your needs/wants.