I had an 89 Ford F350, I had the same problem you had. The shuttering was very bad, I thought it was my transmission too. My mechanic looked inside the tranny & he discovered the universal joints were bad on the drive shaft. That might be something you want to look at, cause that'll give you that real bad shuttering. It took about 1 1/2 hrs to replace them, and that bad shuttering went away.

 

Just in case you did not catch all that was posted, I had a brand new drive shaft custom made with all new u-joints to boot. I drove 250 miles that day and the shudder was never fixed then. I have since replaced or inspected everything but the converter, and now knowing about the type of fluids that was just put in (and will replace that today), I will know by Tuesday what is up with that! I have my fingers crossed after spending over $3,000 the past 2 years and it still does the shudder butt!

The Wiz!