I have a 95 used only for towing. The 5th gear retaining nut came loose at 35,000 miles. It was upgraded then and now has 165,000 miles. Never had a problem since. A friend of mine specializes in dodge diesel truck hop-ups. He states it is a harmonics issue when this trans is run at low RPM in 5th gear behind a Cummins. This is why during my 26 years as a Chevrolet dealer tech I have not seen a 4500 trans failure. My sons 2000 did break the output shaft. It is hopped up pretty good. However when disasembled the shaft appeared to have been cracked half way thru since new. The crack was dull in color. This occured at 170,000 miles.
Doug