Several things to consider:
1. If you could "slow down to 50MPH and get a good ride" then that highway wasn't "too bad".... We've been on some interstates ((I-75 west of Eisenhower Tunnel) where even at 25MPH, the truck/trailer were taking a beating...
2. Longer wheelbase tow vehicles "typically" ride smoother than short wheelbase vehicles, but "typically" doesn't mean "in all situations"... Sometimes there's a situation where the distance from the front truck wheel to the rear truck wheel "sets up a bucking motion" sometimes, that distance is from the rear truck wheel to the front trailer wheel, sometimes it's the distance from the front truck wheel to the rear trailer wheel... Essentially, if the "joint spaces on the road match the spacing on the rig wheels (in most any combination) you'll get a bucking motion. You may find (hope not) that going from a 116" wheelbase to a 159" wheelbase doesn't cure the bucking, but moves the spacing from "one set of wheels to another set" and can make the motion worse rather than better.... On the other hand, if you don't "match the road joint measurements" then you may have cured that issue...
3. Depending on your specific tow vehicle, trailer and hitch settings, you might solve the bucking motion without buying a new truck.... Every setup is different, every setup needs "tweeking" and every setup changes with nearly every trip, depending on what's in the truck, what's in the trailer and what road you travel....
I don't know of anyone who has towed for any great length of time that hasn't found "that one road" where our rig just won't be comfortable... For us, it is the Morganza Spillway... My DW suggests any route to avoid that "monster" and every time we cross it, she suggests where I might want to go for taking her over it "again".....