And 4bt's seem to be a common swap into a Durango. Alternately, the Isuzu 4bd1t (virtually a 4bt clone, only better) are a little cheaper and more plentiful than the 4bt.
Best years for the 4bd1t ar early 91, and older, after Early 91, they went backwards in technology, and made them IDI.
Isuzu is quieter smoother and more economical than the ********.
ALso, it is against Federal law to install an engine older than the vehicle it lands in, e.g. a 1990 Isuzu 4 popper in a 99 Durango, vilates Federal Emissions laws. Dpending on where you live, maybe an issue, maybe not.
Watch Co-Part Auto Auctions, might be worth the time and $ to get registered as a buyer, or find a local car lot who will buy for you, total auction fees plus 10%...
Then you have a wealth of engine choices, start looking at wrecked rigs, seeing what motor is listed, and then googling it/them to see what your looking at and its relative feasibility.