After having done this transmission service on my Jeep Grand Cherokee with the ZF 8HP70 transmission I can tell you this.
1. Do not fully remove the fill plug, make sure it turns before draining. Or fluid will pour out of the fill plug if removed, engine off.
2. Once drained you can now remove the fill plug.
3. The transmission will take around 2 quarts then start dribbling out, at this time you start the engine and put in the rest of the fluid.
4. Look up the procedure for checking the fluid level, need have trans temp 100-120*F with the engine running remove the fill plug.
5. Jeep/Dodge typically use a thermostat in the transmission cooler lines, recommend rechecking the fluid level a day after changing trans fluid, after it has accumulated 20-30 miles and been over the thermostat level of 168*F, then fully cooled, mine took two extra pumps of fluid, just under half a quart.
I used Maxlife fluid, took 5.5 quarts for a drain and fill, 8HP45 in your Durango may take 5 quarts, slightly smaller trans pan.
I'd recommend replacing the transmission pan at 100k, there's magnets inside the trans pan that are pretty gunked up by that point.