Are you using the Merc manual? According to my infor there is no jam nut above the coupler. That one should just be a splined coupler between the upper and lower shift shafts. Remove all the bolts from the gearcase and lower the gearcase. (Make sure you remove the trim tab and the bolt below that too!) If it doesn't lower, a little gentle persuasion with a prying tool may help. Otherwise, soak the coupler with a good quality penetrating oil (I use PB Blaster) and let it sit overnight and try again.


Good luck!