does the Idle Air Control have anything to do with shifting and vacuum?

 

no.
1-2 and 2-3 shifting is controlled by fluid pressure inside trans and throttle valve (TV) pressure from throttle body cable.

older 80's and back transmissions needed a vacuum source, but Magnums don't.

IAC has an electrical connector only, receives instructions from the PCM, and decreases or increases the air gap around the closed up throttle plate to achieve a slower or faster idle.

 

No but your MAP Sensor readings do and controll shifting to a degree (Causing it to downshift and such based on Manifold pressure)

I have a 44RE and I will tell you some things I did and You can compare and see if they match any of your Symptoms.

1. 2-3 Shifting (Vary -like every 3rd or 4th time that I would start from a stop while driving) Would feel like its hunting the gear; Shift; Shift, Shift.

-I replaced the Governor Transducer (Soloniod) and the sensor with Mopar I found on Ebay new for the pair $99.

Fixed that problem.

2. The truck when turning lets say around 10-15 MPH felt like it had no power; had to romp on the gas putting it in passing gear more or less. Same thing going up hills; had to drop it into passing gear.

I replaced the MAP Sensor with a Delphi MAP Sensor and now it shifts down going up a hill or when turning dropping down and getting up and going..