It's very possible the modulator just isn't hooked up. There's not many GM automatic transmissions w/o a modulator (overdrives like TH700R4/2004R). I would run a vacuum line to the modualtor after it's been checked for being bad/leaking vacuum. The detent (kickdown) cable can be left disconnected if you want to shift manually. If the whole cable's missing you'll definitely
want to plug the hole in the case where it enters to keep dirt and water out.
info on ID'ing your transmission.