My buddy had a 3.9 in a long van, it pulled around town speeds actually pretty good, and was reliable as a hammer, he never had to work on it. I never drove it at highway speeds.
I had a 94 3.9 5-speed with 3.55 gears in a short box dakota standard cab, it was a good running truck, bought it slightly used 10K miles, and drove it to over 100K. It was a good little truck.
I had to replace two parts, the entire time I owned it, a water pump, it never went totally out, but starting weeping, so I changed it. A fuel pump, I really pushed the fuel pump, because if you had over a 1/4 tank of gas it would run fine, let it get lower than that and it would start starving for fuel, I guess the extra weight of the +1/4 tank of fuel would keep the pressure up.