1995 Subn A/C Tranny electrical short

Have a 1995 Chevy Subn 1500 4WD with 350 engine and 188k on it. Seems like it lacks Amps and or Voltage to the (this is where I need help) causing the following to happen. Voltage gauge on dash tends to read low around 13 volts or lower-when I test at the battery I have 14.5. The transmission chatters and whines (esp. In the lower gears) until you get up to speed if you have the A/C on-if you turn it off the noise stops instantly. When you are in highest gear you can turn the A/C back on and it will work fine. Otherwise the truck runs great with plenty of power and normal shifting even when towing. I also think the truck cranks a bit slow like it has a weak battery but putting in a new optima did not change this. I'm a lifelong mechanic tricked on this one, looking for a GMC guy who has been there on this problem. Thanks