What @
TBone550 already said about ensuring good trans cooling, trailer setup, weight creep on trailer load, etc is all 100% true. The issue isn't just the trailer it's also the aerodynamics of the trailer, at speeds above 55-65mph the air starts to become a real jerk about things, and above 85+ it's a straight bastard. I also really can't overstate the importance of proper transmission cooling when towing at speed. If you haven't gone to an upgraded trans cooler yet, that is absolutely worth doing no matter what.
As far as the easiest path forward - gearing down would help somewhat, but I don't think it'll completely solve your 55mph up hills dilemma. Ballpark it'd make it a 60-65mph thing at best. That generation of Dodge auto trans doesn't have the greatest reputation for taking the additional heat/load very well either. Getting the gearset, having it installed, and then getting a Diablosport or SCT tuner to uncork a little HP and correct the speedometer is going to run you $1500-$2k ish.
If I'm being 100% honest, having been down this road professionally with LS swaps, turbo setups, superchargers, you name it - unless that truck was a thank-you gift from Jessica Alba for letting her into your bed, best to clean it up, sell it, and buy a different/newer truck that's more suited to towing that kind of trailer.