I looked at this for a long time a few years ago for my 86 Toyota pickup. I researched TDI swaps for a couple years but ending up going for a 5vz swap and am happy with my choice. I think in a 4Runner you would be disappointed with power, a early 2000's TDI has very similar peak torque and horsepower specs to a 22r, the power just comes on lower in the RMP range obviously. A third gen 4runner is way heavier than these early trucks and would probably feel underpowered for highway use and the mileage might not be as good as one would hope. The newer Toyota 4 cyl diesels would be awesome, D4D etc., but are expensive to import and there is no parts network in North America that I am aware of.
The cummins option might be expensive but having a brand new motor with US support would be worth a lot. If you are serious about doing a diesel swap that is the direction I would suggest.