Mariner 60HP - Stiff Heavy Gear Shift

[7089police]

Hi there, Very new to boats in general, I have a Mariner 2cyl 60HP L/S it has the following numbers on a rivited plate - 675_L3107 then on a silver sticker 5486, any idea what year?, it runs and starts very well but has a problem with gear selection, it is slightly stiff/heavy shifting into forwards and almost impossible to get int reverse wthout a lot of effort (dont want to break anything). I have removed/adjusted the cables and also tested then without them connected, cables apppear free, the shift linkage with the cables off of the engine when operated by hand on the slide is still very stiff. Reverse lock is in place but does not appear to move when reverse is being attempted passing from FWD through Neutral.

Can I drop the LU without any specialist tools? if so what am i looking for once i get inside, any ideas whats wrong? Allegedly the motor was shifting OK by previous owner so no real history. LU oil clean and not contaminated and no nasty noises when running FWD.