John,
You may already have done this but.....
Check the shift Linkage/Cable.
Disconnect it:
Use Park Brake, blocks, anytihng to keep it still, and
manually shift the leaver to D, R, whatever, and see if it engauges.
Check Motor Mounts, maybe they're loose/weak & torking the linkage
where it's upsetting the manuual directivity.
Fluid Level,
If it's pretty old, one of the Fluid Solenoids may have stuck.
Sometimes you can add some Additive to the trany,
follow directions, let it circulate, and it may break some carbon/whatever
loose, and get it going again.
Good Luck.
Have a good Day !
S.W.