Overdrive seemed to be not working

The best clue is it was sitting for quite a while. Check for free movement of the throttle cable. When they stick, it is in the open-throttle position. That tells the transmission you're under hard acceleration and it will delay the shift points to much higher engine speeds.

That's the common symptom. Is that cable free?

The first time this happened at the dealership, the transmission specialist wasn't expecting to find a problem with that cable, so he overlooked it and wasted half a day trying to figure out why it was staying in first gear. After that we saw this quite often, but most of the time the cables were merely sluggish, not stuck tight. It's when they sit for weeks with no movement in that cable that they become tight.

When you watch down by the left side of the transmission, you should see that cable sliding freely when a helper works the accelerator pedal. If you aren't sure, remove the cable from the lever on the transmission, then drive the truck to see if it up-shifts. It will shift too soon and line pressure will be low, so don't get on it as it will cause the clutch plates to slip and overheat.

If the cable is not the issue, you'll need a scanner to see what the computer is seeing and reacting to. Most aftermarket scanners will show which solenoids are activated. If there is a problem with a solenoid or its circuit, that should be detected, and the appropriate diagnostic fault code will be set.

The only other thing I can think of is if the valve body has varnish build-up that is preventing the shift valves from moving freely. I had this happen on my '88 Grand Caravan right after it hit wide-open-throttle for the first time in years. The throttle valve got stuck in that varnish that accumulated in the part of the bore where it hadn't traveled to in many years. That eventually cleared up on its own, but it took about six months, and just got better and better the more I drove it. At first it caused the up-shift to second gear to occur at over 35 mph under light throttle, and it down-shifted from third to second at around 20 mph when coasting with closed throttle.

Rats. That was a common problem, and I hoped it was going to be the solution.

Don't be too quick to blame the aftermarket parts. Very often only one company makes a part, then they sell it to other retailers that put their name on the box. For example, if you buy a fuel pump for a Chrysler product from Napa, you're getting one made in the same factory as made them for Chrysler.

Have you checked for any diagnostic fault codes yet? Do you know how to do that yourself? Chrysler made doing that much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds, leave it in "run", then watch the code numbers appear in the odometer display. You can go here:

https://www.2carpros.com/trouble_codes/obd2/p0400

to see the definitions, or I can interpret them for you. This only displays fault codes in the Engine Computer. If you only get a code P0700, that means there are other fault codes in the Transmission Computer. You'll need a scanner that can access that computer to read those codes.