Naturally aspirated diesel pickups died in the early 2000's. They were gutless but extremely reliable due to no turbo, SOHC 8 valve heads and mechanical fuel injection. Perfect for a farm truck, but not good on the road at speed, due to the lack of power. Finding these trucks in good shape might be difficult now.
The newer generation trucks (mid 2000's +) all moved to Turbo intercooled diesels with common rail fuel injection, DOHC 16 valve heads etc. These are a lot more expensive to repair than the earlier generation, however they have double the power, so perform a lot better on the road.
My experience with mid to late 2000's pickups from Toyota, Mitsubishi, Isuzu and Ford (Mazda B series based Ranger) was good, they were all pretty reliable, at least up to 10 years old.