1999 25 hp tiller won't go into revese

[IH8JEEP]

I had my 1999 merc. 25 hp tiller that was in the shop the other summer for a water pump install. The motor seemed fine but I had no reverse lock. The shop T&S marine in New Oxford Pa closed up and went to Florida(BEWARE). I couldn't take it back to get thier work fixed right. I just used it this way and only used reverse very little and very,very slowly to back out of dock. This year started using reverse a lot and under more power. Reverse started to get harder and harder to engage. I would have to "force it"(turning harder than comfortably) with the twist grip to engage. Got harder and harder each time till it wouldn't engage. I checked all cables and linkege they were good. The shift rod seems to get stuck/tight before engaging. Motor still runs great in forward and no metal in gear case thank goodness. Any idea whats causing this? Thanks