Drivetrain Transmission Not Engaging

[nero6]

I had my mustang out a few times this year with no issues until now. I took it out of the gargae and parked it in the driveway for a few hours, no issues when i did this. A few hours later I went out to move it back into the garage (car was parked on a slight incline), the transmission lever would go into each gear (C4 Transmission) but no gear would engage, the engine would only rev. I assumed the transmission fluid was low, so I added 1/2 litre. Took it out for a drive and there were no issues. Came back to check the transmission fluid and now it is reading over full, so the transmission fluid was not the issue. Any suggestions on what the issue could be? It seems to shift fine now.
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67 Hardtop
Dark Moss Green Metallic
289 2V V8
C4 Automatic

 

[woodsnake]

Perhaps unlikely, but maybe shifter linkage under car is not tight. Or, sloppy shifter bushings not allowing rod to actually move..

 

[Falcon79]

Maybe check the vacuum line that runs from the intake manifold to transmission.

 

[nero6]

Thanks for the suggestions, I will take a look at both.

 

[candphall]

I'm with Woodsnake I'm betting n a linkage issue.