Many thanks for raising this problem. Yes, I've had similar issues. I'm driving a 2016 SR5 4x4 Tacoma carrying a 2016 Fleet with a delivered weight of 1300 lbs. I carry minimal and only very lightweight gear so far (i.e., think lightweight backpacking gear and a few flyrods) and my wife and I are not big people. However, i don't know my loaded axle weights or overall weight.
In preparation for carrying my camper, I installed Firestone air bags, Bilstien rear shocks, and E-rated tires. My empty truck was almost undriveable on some segmented concrete freeways and highways on the 2,200 mile trip from Florida to Arizona to pick up my camper last spring.
It drives much better loaded, but I still encounter dangerous harmonic front to back bucking on some particularly uneven segmented high speed concrete roads. I've tried different speeds and air pressures in my tires and air bags, but while much better on normal segmented roads, the problem still persists on bad sections. Driving at slower speeds and reducing the air pressure in the air bags helps. I typically drive 55-60 miles per hour on highways/freeways. I go out of my way to avoid interstate freeways and some state highways by preference, but sometimes it's unavoidable. I put 12,000 miles on my camper/pickup last summer and haven't experienced any other handling issues except for this problem (e.g., it corners fine, handles 40+ mph sidewinds and passing trucks with ease, etc.).
The on-line research I've done suggests that installing adjustable rear shocks might be the answer (e.g., Boss). My camper is mounted only part time and I use the truck for hauling other stuff. I'll be very curious as to what other's recommend.
