Oh gurus of all things suspension... Help!

Ok, so i buy this 97 dodge 4x4 extened cab sport. 111k on the odometer(360cid magnum, auto trans), body straight, paint good, engine runs albeit a little rough. Put in cap, rotor, wires, plugs, O2 sensors (both),
PCV valve, flowpro catback dual exhaust and a new set of 265/75/16 "more highway than offroad" tires. When I am going anywhere between 40 and 70, there is a shimmy (it's severity comes and goes with no rhyme or reason as to under-power/coasting, engin rpm or speed although it is somewhat more pronounced under load) that feels somewhat like a wheel out of balance or a bent rim. I took it back to the tire dealer, (Discount Tire) and they put all 4 on the road force balancer and found two of them to be out of spec so they replaced them. The other two balanced out a zero, and none had any rim problems what-so-ever. I am pretty confident that the problem is not in the tires or wheels. The sterring is still very tight with no wander or clanking and the vibration is throughout the turck and not in the steering wheel. What should I be looking at to find the cause of the shimmy? It gets bad enough at times (like triving on a washboard) that I feel ilke the truck is gonna come apart.
Thanks in advance,
-Jerry
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I just wonder if maybe you have a U joint problem, or slightly out of round drive shaft? Or U joints that are starting to go? Thats my 2 cents..
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