Sounds like your bottom shift cable has gotton some wear in it..( the cable that goes through your transom to your outdrive) they get burned or kinked sometimes with all the different mechanisms in that area...it is a real bear to change unless you have a special cable tool to use..but it can be done...other than that it could be low fluid, or fluid that is contaminated with water....1st thing I would do is see if that lower cable is noticably worn...then pull the outdrive and see if that tight feeling is still in the shifter....it is not necessary to pull the drive to replace that cable, but you will need the boat out of the water...