owned an 82 johnson 175, pretty much the same engine.
check the shift rod if you have had the lower off, they screw in and 1 or 2 turns will mess it up.
check linkage at cowl, cable should not slip at all in bracket that clamps it, may just need to compensate for an older shift cable that has stretched some.
with key out put shifter in N and turn prop, do you hear any noise? put it in R and GENTLY try to turn it but be very careful not to turn engine over, just check for resistance to turning (in gear it should not turn easily).