The 5.7 diesel used a much thicker casting block, (DX is cast into the sides)
Not all 5.7 diesels are DX blocks. The early D-blocks (78-80) weren't, and they had a lot of the problems associated with the 5.7 diesels. The DX blocks are better quality though.
The DX blocks were introduced in 1981, they had 3" main bearings and roller cams/lifters. By 1983/84 the 5.7 diesels were fairly decent engines, they upgraded a lot of the weaker parts by switching to the DX block and had added a fuel/water separator. But by then they had earned such a poor reputation, that it ruined the reputation of almost all passenger/light duty diesels in the US.
The lack of a fuel water separator, poor fuel quality, weak head bolt design (and too few head bolts), people driving and maintaining them like they were gassers, and improperly trained mechanics were the biggest problems.
Mechanics would reuse the over-stretched head bolts in engines that came in for a "bad head gasket," and then in a few thousand miles the head bolts would break or not be able to keep the head tight, and it would leak again.
Many people forget about the 80's LF7 4.3L V6 diesels (found in GM A-body cars). I've seen one. It was in a 1985/86 Olds Cutlass Ciera or Buick Century (one of the two).
I wish they were better engines, and that they didn't kill the diesel reputation in the US. I want the US to be like Europe/Brazil. Almost everything with wheels has a diesel option for a powerplant. Maybe if that were the case we wouldn't have $5 gas, SUV's getting 9 mpg, and so many power/efficiency-killing, emissions controls. Instead we'd have high-efficiency (and high-powered), 30mpg SUV's and 55+mpg cars (maybe even diesel motorcycles), a good bio-diesel industry. And the 6.2's would get more love.