Attention: TDR Forum Junkies 
I've been getting the TDR for about a year, but I'm new to the website. I'm looking for assistance with a problem. If I'm going about this the wrong way, please forgive me and steer me in the right direction so that I can get some potential solutions to my problem. I've had a shimmy in my 1998 2500 diesel club cab (12 valve,automatic) for about 9 months. I can feel a vibration in the chassis (seat of my pants), but not in the brakes or the steering wheel. I can feel a vibration similar to driving on a washboard road at about 42-45 mph. At any other speed (while driving) there is little or no vibration. I can also feel the vibration while braking, but only at certain speeds (around 40, 30, and 20mph )and then, only for a few seconds. It will shimmy at 40, smooth out, shimmy around 30, smooth out and then again around 20 mph. Sometimes the shimmy is very pronounced and other times it is light.
I've had various opinions as to the cause of the problem. One dealer mechanic thought it was brakes, but the shimmy is not consistent and I cannot feel a thing in the brake pedal. I've looked at the rotors, drums and pads and they look okay. I know this is not a very scientific way to determine if there is warpage.
In don't THINK the problem is wheel balance or out of round tires. Since the problem started, I've replaced 2 tires and had all 4 rotated and balanced. I just had the front end alignment checked last week and the mechanic said the alignment was within specs and the front end parts were in good condition. I might add that the truck is a 98 but only has 41,000 miles on it. After checking the alignment I had the tires rotated and balanced again, but the problem still persists.
I'm no mechanic, but my intuition tells me the problem may be in the transmission. I had the transmission fluid changed about 10,000 miles ago. I ensured they used the correct type of fluid and the shop later added a 1 quart additive that is supposed to make the fluid more viscous (is that the right word for more slippery?) This had no impact on the condition.
Do the 98 automatic transmission have a problem with the torque converters not unlocking? The dealer mechanic didn't think this was the cause of my problem. He said the problems they have seen with the torque converters have not caused this type of problem. If I coast to a stop I don't get the shimmy, so that makes me think it could be brakes. Is there some type of transmission sensor that is liked with the brakes?
Has anyone had a similar problem and if so what was the cause. Any help is appreciated. I don't have money to blindly throw at the truck in hopes of stumbling on the right solution.
Thanks
Gary
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