As I recall, the front-frame-flex/break was an issue even on the D150 gas rigs. Perhaps an additional trans cooler can help trans longevity a bit?
From what the guys at our sponsoring Mopar club dealership told me, when the diesels were being devoloped, they got the biggest-capacity rear axle at the time (Dana 70, I believe, which GM later used too), but they still had the option of "Break Axles or Break U-joints". They chose u-joints to be the weakest link.
Then, too, I had some confirmed sightings (by one club member) of an older square-body Cummins Dodge, pulling a 60' loaded cow trailer, starting from a red light on a slight uphill grade, and it pulling the fornt wheels off the ground to get it moving. Never saw a Ford or GM truck do that. Must have been the Dodge Race Heritage at work? Or a frustrated drag racer driving it?
If it doesn't have a Racor (or similar) water separator/filter in the fuel system, it probably needs one.
Enjoy the truck!
CBODY67