Your shift linkage rod from the floor shift to the side of the gearbox sounds like it needs adjustment.
place car on hoist or ramps. put shifter in park.
loosen jamb screw or nut on shift rod.
verify linkage on gearbox in park. park pawl locks on.
retighten.
This can be done or checked in all gears.
fairly easy to do.
check your bushings in the linkage and inspect your trans / motor
mounts for collapse. they can cause this too.
DR.D