Small diesel pickups were sold in the US by a few manufacturers in the late 1970s and 1980s; but they were not popular; and were often poorly maintained; as most mechanics were not familiar with them. Some of these diesel models were sold by Mitsubishi and Dodge from 1983-85 as the Mitsubishi Mighty Max SP, or the Dodge Power Ram 50; Ram 50 Royal; Ram 50 Sport. These vehicles had 2.3 liter motors, and some were equipped with optional turdochargers.
Isuzu and Chevrolet did the same thing during the 1980s; the Isuzu diesel pickup was sold from 1981-88; while essentially the same diesel powered vehicle was also sold as the Chevy S10 from 1983-85. They had 2.2 liter motors, and the 1986-87 Isuzus had optional turbochargers.
Ford Rangers had optional diesels from 1983-87. The 1983-84 models had 2.2 liter motors; and the 1985-87 motors were 2.3 liters with turbochargers.
Volkswagen also made small diesel powered pickups, which got exceptionally good fuel economy; but they were so underpowered that they soon developed a terrible reputation for their lack of reliability.
International Harvester also made their Scout II with an optional 3.2 liter I-6 diesel from 1976-1980.
I feel it would be impractical to buy a used diesel vehicle that old, and expect it to give reliable service when towing. But if you were enterprising, and willing to invest the time and effort; it certainly would be possible to find one of those old pickups with a body in good condition, and install an appropriately sized new Kubota diesel motor in it. I feel that Kubota diesels are the best quality engines available for that type of service. They are sold in the US, primarily for industrial and agricultural use. My preference for this application would be their engine model V2607-DI-T-E3B. This is a 4 cylinder, 4 cycle, liquid cooled, 2.6 liter turbocharged motor; which produces 66 hp, and has a maximum speed of 2700 RPM.. For such a conversion, it would also be necessary to have motor mounts fabricated, and an adapter plate between the motor and transmission; plus a means of attaching the flywheel to the clutch pressure plate. But you would then have a vehicle which was simple to maintain, smog exempt, and free from complicated electronic emission controls.