You can buy a brand new 1/2 ton pickup with a 3.0L diesel from Ram, Ford, and soon GM. If that is what you want.... and you can afford it.
The biggest issue with "American" full sized pickups, even the 1/2 ton variety, is their portly curb weight. They have so much of their own bulk to get moving even when empty, that they need quite a bit extra to be able to haul or tow anything significant without complaining. And what you can tolerate is of course your own.
I have a 1996 F150, with the standard issue 4.9L I6 bolted to a proper manual gearbox. 2WD, regular cab, long bed. That engine has 145hp and 250tq, lots of oomph off idle, and does not need (well, it really can't) to spin over 3500 RPM. It mirrors almost lock step with the 2.0L BHW TDI, which is 136hp and 247tq, and the operable RPMs are pretty close too. I think that would maybe be a decent swap, but only for whatever uses the standard engine would be used for. I'd never pull much with it, or haul anything over its meager payload.
Oh and the Fiat Ducato (Ram Promaster here) gets a big 3.0L Iveco 4 cyl diesel (option).
Plenty of other examples of 4 cyl engines (diesels) tasked with a big job yet able to get it done without any troubles. My 2.1L Sprinter is perfectly adequate, and I am sure the 2.8L Express/Savanna is too. With the right gearing, you can do quite a bit. Just depends on how fast you want to go and how quickly you want to get to however fast you want to go. VAG uses 2.0L 4 cyl TDIs in their Crafter vans. Ford uses 2.2L 4 cyl diesels in their Transit vans (we only get the 3.2L 5 cyl or a pair of worthless gassers). But those vans have a much more efficient unibody, so their curb weight isn't sapping so much power.