Chevy transmission wont shift

I have a 77’ chevy pickup with the 250 ci. inline 6 and a TH350 auto. The transmission is not very old, but it does leak a lot. I keep it filled and haven’t had any other problems until today. The only way I can get it to shift is by shifting into neutral and then back into drive in between gears, otherwise it just stays in first gear. Any help is appreciated.

Possibly a sticking governor. These are rather simple to check. Pull the round cover on the outside of the trans and check it out. I would also get those leaks fixed. Leaky transmissions have a tendancy to burn up.