In addition to checking the clevis pin is free, I'll suggest checking the cable isn't getting kinked somewhere when it's being operated, and that the transmission end of the cable is aligned to the shift lever. It sounds like something is binding when the cable was attached to the transmission.
Although its unlikely the cable is the problem here, I'll also go with the advice to replace a corroded cable. WD-40 is fine for helping unstick things, but it completely evaporates in no time. If water was getting into a cable to make it corrode in the first place, it will keep getting in and that cable is going to seize again before long. You're still going to replace the cable, it's just a question of how many times it will fail on you before you do.