A diesel 4Runner is just not going to happen anytime soon, unfortunately. Toyota had been working on a diesel engine for the Tundra, but they have publicly announced that they have shelved the project. The sales of the Tundra far outstrip the current sales of the 4Runner and the pickup market is more amenable to diesel engines than the mid size SUV market. So if Toyota figures it doesn't make economic sense in the larger pickup market, I'm guessing that it would make even less economic sense in the SUV market. The 4Runner is now a niche vehicle. A diesel 4Runner would be a niche within a niche.
Developing a diesel engine just for the 4Runner does not make economic sense for Toyota. When word came out that Toyota was working on a diesel engine for the Tundra I thought GREAT, Toyota is smart enough to design an engine that could be used in a couple of different models (Tundra, Sequoia, 4Runner and their Lexus equivalents). That would spread the R&D Costs around and allow for the sharing of the same parts and training, etc. Yes I do realize that the engine certification costs could not be spread around. Apparently Toyota is not as smart as I thought they were as it appears they were working on a model specific, application specific, engine which in today's automotive world has a LOT of disadvantages.
While a torquey diesel would help towing capacity, I'm not sure it would give that much more towing capacity over the V8. Towing capacity is not just governed by power, but also by frame strength, wheelbase, and braking. I wouldn't be comfortable towing more than 7000 lbs in a vehicle the size of a 4Runner. YMMV.
I agree that the towing capacity of a vehicle is not determined just by the engine in the vehicle but more by it's frame strength, wheelbase, and braking.
The VW Touareg is a prime example of that in that it's tow rating is the same for the V6, V8 and V10 Diesel engine models (not sure about the new V6 Diesel engine). The BIG advantage of a diesel engined 4Runner would be the MPGs both when towing and when not towing. In both cases I would do a lot for more than the 7-9 MPG when I tow and the 19-20 when not towing.
The current automotive market is all about finding a niche and owning it. Right now, the mid-size SUV market has no diesel options. The 4 Runner is the defacto leader in this segment amongst the enthusiast end of the market. That enthusiast segment is also pretty interested in diesels. Ergo, I could see it making a whole lot of sense.
FYI, the VW Touareg mid-size SUV has had a diesel engine off and on since almost the year it was introduced. Granted, the Touareg is a "Luxury" SUV which their owners like to point out to me when I get into discussions with them. Recently VW retired the V-10 Diesel engine in favor of the V6 diesel and also has/will eliminate their V8 gas engine. The only problem with the V-10 diesel engine VW Touareg was/is the $65-75K cost. Not sure what the price of the V6 Touareg runs these days.
The full size truck market is really REALLY competitive. Toyota is getting their clock cleaned. For all their ills, the one area where the Big 3 know how to compete and kick some is in the big ass truck segment.
Yes, the Big 3 do sell trucks. One reason they sell trucks is the DIESEL engines they offer in them. When it comes to people who do a lot of towing, the VAST MAJORITY of truck owners have a DIESEL engine. If/when Toyota gets SERIOUS about competing in the truck segment (and other segments as well) they will have to man-up and offer a diesel engine in their trucks and other vehicles.
BTW Toyota, if you think putting Hybrid technology in the 4Runner in the future will keep me as a customer you could not be more WRONG. Give me 7K pounds of towing capacity and an engine that shows me it is capable of pulling that weight. Hopefully, I will get another 4-5 years out of my 4Runner. That is, if I can keep winning the battle with RUST I am fighting. That is a different thread for another time.