I have the 5.9 liter 1998 durango with 150,000 miles on it and just went through the same problem. It had to finally die permanantly and get towed to figure out the problem. It was the crank shaft sensor that went bad in my case. The symptoms were intermittantly dieing on the highway, but were not limited to inclines, declines, etc. The engine had to be under heavy load, towing a boat, to finally have it die. Aperantly, the crank shaft sensor, when it goes bad, is sensitive to heat, so it only happens under load which gives off higher heat in the engine compartment / transmission. The sensor cose about $150 to have the dealer replace. I have had not had the problem since.
Hope this helps!