Hopefully it has 90 wt gear oil in her. If so, then remove the 8 bolts off the top loader and take it off for inspection. Should be able to move (actuate) the shift gates on the 1-2. and 3-4. Then if that checks out take a look see into the gear box for any carnage.
I recently saw a guy that had a trans that would act up like this when it got warm and it was really low on 90 wt. and the shift sliders turned blue because of heat.

Just saw Grigg's post too, so he mentioned fluid level too, a bad seal in the torque tube can drain a transmissions 90 weight into the rear axle housing starving the transmission for fluid at the same time over filling the differential.

Last edited by sstock; 07/20/2018 7:54 PM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration