Heres what happens:

When going 40-60 and I let my foot off the gas, I can feel a significant decrease in speed. I obviously know its supposed to slow down because of my foot releasing the throttle, but it feels like im actually applying the brakes a little (i know im not)

also, sometimes when I throttle it lightly right after taking my foot off the throttle while cruising, i can feel a shake while i hold the pedal down. any ideas?

sometimes black residou shoots out of my exhaust at startup, im pulling a front o2 code. could this be it? or is it probably something different?

 

Durango's are bad for missing, hardly do u find a 98 thro 00 that doesnt have this problem, first off the slowing down is because u dont realise how heavy durango's are, they weigh over 4k lb's.. alot ALOT heavier than a car an a truck, mine does the same thing, takes forever to get from 0-60 but no time at all to slow down.. u could also check your tire pressure, this will also make your truck lag alot. As far as the smutt from the exhaust, mine also does that.. it's just exhaust that's built up inside the pipe gettin blown out, all cars do it after about 60k mile's

 

though, this doesn't sound like a tranny problem, right?

 

Kinda sounds like your torque converter isn't disengaging from the engine right. Have it looked at. As for the black smoke, it is unburned fuel. Give your truck a tune up with some OEM Champion plugs or better yet Champion Truck plugs. Also replace your O2 sensors. Double check your air cleaner.