Just my .02 cents, but Walt (TAVLLC) and I mapped out a diesel swap on my truck with the "at the time unannounced" Cummins 2.8. The 2.8 made more sense because we wouldn't have to completely change out the entire drive train to make it work.
I think we were looking at like 30K+ to make that happen, and the reality of it is that both he and I decided it wasn't worth it. I mean, if you feel like that kind of money is no issue, then rock on with your bad self. But there are some major issues besides the cost and time.
Someone mentioned the toyota diesel, and the numbers on that engine suuuuuuck. You also have a major issue, in that if you break a part getting a new Toyota diesel engine component could take weeks or months to source. Whereas a Cummins component is literally over the counter.
The Cummins option is great for vintage FJs where engines have died and are harder to source, and you don't want to go with the LS swap route, which admittedly is kind of awesome if you've seen Tim/Sackwears FJ45 conversion. It just sounds awesome.
So besides cost, the thing is, I'd rather just have another platform. If we're talking 25-35K to do a diesel conversion, I'd rather just buy a ZR2 or even sell my 4Runner and buy a raptor and bomb over whoops with a 450hp fullsize truck.
This next statement is sure to ruffle some feathers: The other big hold up is that I just think dropping a new engine in the 4runner is like pearls before swine. Lets be absolutely honest with ourselves, the 5th gen will never be a "classic." Almost all cars produced after the turn of the century, even in the 90's onwards, are all forgettable modern design, with the exception of a rare few. I think a clean 3rd Gen 4Runner might be on the bubble, but the 4th gen is completely forgettable. Sorry they are...and so is the 5th gen. It's a cool looking vehicle, but just like light bars and whatnot, the design winds will change and it'll get lost to the sands of time. It's kind of a reason people love the FJ40, because it's classic despite kind of being a turd to drive. And I say that as an OWNER of an FJ40 that's currently sitting in a warehouse (in parts) in the bay area. I just don't have the budget to rebuild and modernize it in the style and level of my desires.
So to me effectively putting the entire cost of an engine swap into a platform I don't think is going to become a classic just becomes a case of "I just want to do it and burn money." To which I'd say, "Cool story, bro!"
I dumped a stupid amount of money into my truck, but at some point you have decide if the juice is totally worth the squeeze or if another platform will give you more of what you want without attacking the fundamental strengths of the platform, which in this case is reliability.