Yes, what he is saying is a lower unit issue. Pre-1976 Mercurys are well know for failed lower units, due to rust from water intrusion. The shafts were steel, not stainless; and they pit, tearing the oils seals out. That being said up front, I have never, ever seen the clutch RAMPS of either the gears or sliding clutch WEAR out. I have seen a couple of totally shattered clutches but never worn. Shift issues on Mark 30s are usually related to the shift linkage itself. They shift with a twisting motion. This motion is often lost due to spline wear, incorrect reverse lock adjustment, twist-distorted shafts or all of the above. Sometimes, it can be as simple as someone putting the lower unit on and not lining up the shift shaft splines properly. Best way to tell is to pull the lower unit off and see if it will shift when not attached to the rest of the linkage.