I own a 1999 dodge 2500 ram quad cab diesel two wheel drive truck. It has 165,000 miles and I tow a 34' airstream trailer. When towing up steep mountain grades, out of overdrive, I encounter a shimmy as I apply more and more power. I sometimes downshift to 2nd to relieve the drive train. The problem has become progressive worst over time. I encounter the problem when not towing and going up a grade and take the it out of overdrive to relieve the problem.
My question is, does the problem relate to the transmission and or torque converter or to the engine and would reflashing the computer eliminate the problem or should I look for the remedy elsewhere?
Start by checking the rear transmission mount, then the center bearing support for the drive shaft if it has one. The was an instance where we were told to replace the two-piece drive shaft with a one-piece and no bearing. That solved the problem for a few years at which point we were told to switch to a two-piece again with the bearing when the problem came back. Try finding the right part numbers! This fellow pulled a cattle trailer from Wisconsin to Florida, back and forth.
You might also check ride height to see if the rear leaf springs have sagged. That will change the drive line geometry. The goal is to have a slight drive shaft angle going down where it leaves the transmission, and up from the pinion gear in the axle. If that angle diminishes due to sagged springs, it will cause the u-joints to not rotate very much during each drive shaft revolution. That will greatly accelerate wear in the cups from the needle bearings staying in one spot and being hammered on. They need to be constantly moving back and forth while driving.
You could have nothing more than a tight u-joint too. Look for signs of reddish-colored rust by the rubber seals on the cups. That's a sure sign of dry, binding joints.
If there is a shudder from the torque converter, it might change with load but it will change with changes in engine speed. When the vibration occurs and you can keep it acting up, hold the accelerator steady, then lightly tap the brake pedal. That will cause the torque converter to unlock. It will lock up again in two or three seconds. If the shudder goes away during those two seconds, suspect the lockup clutch is slipping and chattering. Using the wrong transmission fluid can cause that.