2002 shuddering at hwy speeds
2002 FlexFuel 3.0.
Last month while driving cross country my truck started shuddering while climbing a long grade. Since I had it on cruise control I immediately disengaged it and let it slow down. The shuddering stopped. When I tried to accelerate back up to 75mph it began shuddering again. It continues to do this and almost only when encountering a long grade at 70 plus mph. Additionally, the symptoms occur only in the 3200 to 4000 rpm range. If I can force the transmission to gear down and the engine to run above this point it will smooth out and pull as it should. While in Knoxville, TN I brought it by the dealership to have it checked for error codes and to find out why it might be doing this. Unfortunately, what they thought, and I paid for, was not the cure: plugs, wires, intake servicing. It can't be bad gas after filling the tank so many times since then. I'm suspecting O2 sensors at the moment, but realize it might be other problems, too.
I've seen similar symptoms in a few threads here but nothing that fits perfectly. Any ideas on what else to check?