FYI....I have a 3.9L with misfires that I could not figure out as well as stumbling and sometimes stalling etc. Turned out to be crank sensor. Did not get any codes to this effect but after a long battle of replacing sensors and getting bad ones this is what it turned out to be. I was able to run live diags on the vehicle and the only indication I had was the timing was jumping around with large gaps. Not sure what type of timing sensors your truck has but I would look into it.

 

x2 on the crank sensor. It can be a cause of funky things. I had the infamous 1, 3, 5 misfire codes without any actual misfires while driving 70+, if I never exceeded 65, never got a code. When stopped at idle, however, it would occasionally lope and almost die (twice it actually died). Since you've done the basics, plugs, cap, rotor, and O2, I'd say the crank and cam sensors. Your idle range seems fine, my 3.9L idles at around 600-700. Clean your IAC as well, on the back of the throttle body, do not move the plunger, but clean it well, requires torx to remove. The crank sensor is on the back of the block at the transmission bellhousing, on the 3.9L removing the passenger front tire gives direct access, on the 5.2L, it was easier to come down from the top, passenger side of the distributor. The 5.9L should be the same as the 5.2, but check both ways to see what's easier for you. The cam sensor is in the distributor, it's the plastic plate under the rotor with a hall effect sensor, it comes right out after you pop the rotor off. Make sure to get mopar sensors. My problem was the previous owner put in a 3rd party crank sensor, put a junkyard mopar sensor for $4 in, never had a problem again. Hit a junkyard first, the crank sensor should be the same on any 3.9, 5.2, and 5.9 from a Ram, Dakota, Durango, etc. Easiest will be pulled from a 3.9L from the wheel well, 10mm socket and 1/4" drive ratchet, comes right out. Also, make sure it seats right in the rubber grommet, if it pushes it out a bit and isn't seated properly, it'll hit the flex plate and eat up the end of the sensor.

 

 

They might have simply used an inferior sensor from a local parts store instead of an actual mopar sensor. There's a ton of threads about how these trucks do not like 3rd party sensors.