The check engine light combined with erratic shifting is most likely the "transmission output shaft speed sensor," a $12.95 (approx) part that is easily replaced at home if you are OK with dumping some trans fluid on your driveway! The dealer told me he sells dozens of these sensors a week! My OBD II readout pointed to incorrect (or zero, I forget which) transmission output shaft speed, and I figured the only way this could happen was if the sensor was bad. My hunch was right. A new sensor cured my problem instantly.
It is a plastic sensor that screws into the left side of the transmission, near the tail of the transmission, and has a wire harness coming from it. It is below the level of the trans fluid, so fluid comes out when you unscrew it. Maybe if the car were tilted more to the right it would not, I don't know. Good luck.
Radiator drain plug is on the lower front of the radiator on the passenger side --- and you really have to look to find it. It's plastic -- so be careful.
The VSS sensor on the transmission tailshaft is a high casualty item. Almost every post I read talks about very high engine RPM's before the transmission will shift, speedometers that don't work, etc. The VSS sensor is the culprit. And as was earlier posted, it is an easy changeout.
I read a lot about transmissions not shifting until the engine is really revved up. After my '99 Durango transmission failed at 180,000 miles, the transmission shop kept the vehicle for 10 days. When I went to pick it up, I asked if they had readjusted the shifter cable between the throttle body and the transmission. They stated it was OK. When I drove off on the level road, I immediately knew it wasn't. The transmission did not shift out of first gear until the vehicle was going 30 MPH. And the shift was very rough. I finally got into third. When I got home, I popped the hood and found the cable very loose. They, of course, had removed the transmission for rebuild and had simply reattached the cable at the transmission end when they reinstalled it. I reset it per the Haynes manual (easy adjustment). Retried it and was in third gear at 30 MPH and OD at 45. If the cable is too tight, shifting occurs too early and is rough. Also, the slightest increase in acceleration will cause a downshift.
I have a Dodge Durango 99, and it doesn't shift unless I put it in neutral going at speeds usually around 25, wait 3 seconds, let off the gas, then back into drive again, then shifts. Also RPM sometimes goes up to 4000 (4 on the dash) before it shifts into gear, and sometimes 3, it just depends. Obviously something's wrong, as I can hear when the car attempts to shift.. Any help??? THANKS.