Do you have 2 cables coming out of the gear box, or one from the gear box and one up from the outdrive?
Is your unit Merc or Volvo? If you can locate where the cable coming up from the outdrive bolts on to the engine, disconnect it from there and try your shifter. If it moves easy then I would look and see if it is corroded where it connects at the outdrive. I have changed mine, (twice the one that comes out of the outdrive, never the gear box)) but we are 17 years apart and I am sure they have made a lot of changes, so we may not even resemble each other. Mine is Merc.
Please keep me posted, not sure if I am helping or not.