Whine in ‘96 Chevy 1500 5.0L 4l60e transmission.

[Nokgriot]

Hey everyone, can’t pin point this issue. Little back story first. Bought a ‘96 Chevy 1500 5.0L from a guy who said the transmission was slipping. He could drive it from a couple hundred yards then it would slip out. He’d stop it and let it sit, then repeat it. So sounded like to me he was low on fluid and was starving the pump. 
 

I was able to drive the truck on and off the trailer, and into my shop no issues. I removed the transmission and it was about 4 quarts low. Oil looked fine, no metal or contamination. So all I did was drain and refill the torque converter, filled transmission to the proper level, new filter, and new oil pan gasket. Then reinstalled the transmission. 
 

Started it up last night and there’s a constant whine now. I can shift into Park, Reverse, Neutral, and Drive. Whine doesn’t go away but doesn’t appear to get worse. Backs up fine, but in Drive it won’t come out of 1st gear (I’m assuming), just won’t upshift. Also have a light on the dash that says to Check Gauges. 
 

Is it possible the torque converter is shot? Could I have a loose or faulty connector that’s keeping the transmission in “limp mode”? Any other ideas?

 

Thank you in advance!