89 Dakota 3.9 auto. Suddenly the trans won's shift into 3rd. Fluid level is full and clean. No burnt smell. Then when stopping at a light/stop sign it will stumble, but not stall. I remember reading something about the IAC being tied into the trans shifing??

Normal reverse operation ?
4 speed I assume or 3 speed ?

First thing to check if reverse works is the throttle pressure linkage/cable.
Open the throttle engine off to wide open then release. Make sure the throttle pressure cable and lever at the trans returns all the way forward.
There should be a spring on the lever at the trans that returns that lever forward.

If no reverse then things get more complicated.

3 speed with the OD on off button? Reverse is fine.

1st, 2nd, and 3rd are hydraulically controlled. O/D is controlled by a solenoid.

When it won't shift to 3rd, try accelerating a little more and then throttling back to idle; will it shift then?

I tried it, but hard to tell if it shifted. I'm leaning toward saying yes, because it didn't stumble when I came to a full stop.

Thinking front piston seals John?

Front clutch seal/rings or governor sealing rings.

A strong Reverse isn't always a good gauge of front clutch operation; the line pressure triples in Reverse so a weak clutch will often work acceptably in Reverse.

I finally got around to pulling codes and I got a "DTC:15 NO VEHICLE SPEED SENSOR SIGNAL DETECTED DURING ROAD LOAD CONDITIONS". A symptom of this sensor failing are "can cause an automatic transmission to go into limp mode, and not shift above second gear". Think I found my problem.

No you didn't if you are accurately describing the condition. Since you don't have an electronically controlled transmission there isn't a "limp mode". No speed sensor signal will result in no 4th gear shift but it has nothing to do with the 1/2 or 2/3 shift. Please try to accurately describe the condition you are trying to fix if you want accurate information to fix it.

From a dead stop, when I accelerate, is that 1st gear? If so, I than feel 2 more shits after that, and than it hangs up (4th gear?). Maybe I did describe it wrong. I thought these were 3 speed transmissions.

I replied to your PM. Don't want to type all over again, I'm on my tablet.

Got it! Thanks

UPDATE! Sensor installed, trans shifting normally again!