Good discussion. Learning a lot.
I'm not a Diesel expert (yet) but I think one reason Diesels smoke when under load is there is a lag for the exhaust to spin the turbo faster and get the extra air into the cylinders. Just a thought.
So, I concur, there's unburnt fuel coming out the tailpipe. It could be as simple as the lighter fuel components have evaporated and left behind a lot of gunk and gum. I'd say change the oil and oil filter (it's a must do if it's been siting that long--transmission fluid and filter also a good idea) change the air filter, dump in a dose of fuel treatment/injector cleaner and go for a nice long ride.
Fill up with a fresh tank of Diesel fuel and a double dose of fuel treatment. Run it for a while then change the fuel filters.
If that takes care of it, great! If not, dig a little deeper.