What's wrong with my TH350 tranny?

[ssuesens]

Trying to troubleshoot my son's TH350 AT in his '71 C10.
Yesterday it began sticking in gear (2nd gear I believe). After moving column shift to D, it's not shifting up or down and making a soft whirring or whining noise on acceleration. I replaced and checked the vacuum line and verified the vacuum accumulator is getting and holding a vacuum. Also checked AT fluid level and its cherry red and full. Kickdown cable is not stuck or binding. I visually inspected the shift linkage and it appears to move through the motions correctly and Park, reverse, & neutral all are working fine. My next guess is looking at the governor but that's above my comfort & knowledge level. Any tips?

Anything else I can check prior to hauling it down to the tranny shop?
Thanks in advance!