To determine if the problem is binacle/shift cables, shift tube (which connects to the spline on the lower unit), or the gearcase itself, do this. Pull the clevis pin and lift off the shift cable linkage (in the engine pan). Now see if it shifts normally. If it does, the problem is probably the shift tube binding. Right below where the tube connects to the shift cable, there should be a grease fitting. Is it dry? That one is easily overlooked when performing annual maintenance.
If after greasing the fitting, it is stiff (and you have already determined the shifter and cables are ok, when disconnected), then you;ll need to drop the lower unit, and place it in a stand. With the lower unit in the stand, and a small pair of vice grips clamped around the shift spline, with a rag, to keep from marring it, turn the prop slowly, move the pliers, and see how it shifts. It should shift very easy. If it's still hard to shift (unlikely), the problem is in the gearcase.