I would assume that the shift keys or inside of the gears are worn. You could try unhooking the cable to see if a nudge over on the lever helps. When you go inside again, verify that the input bearings are healthy yet. What's the model #?

@tomplum Thanks for the pro tips! Disconnecting the cable and nudging the shift lever did not help, so the shift keys are looking more like the culprit, but now that you mention bearings, those could be making the sound that goes with this problem- a rapid, irregular metallic groaning noise that you can feel through the handle- not at all like gears grinding, more like a bad bearing. Went over every tooth in the gearbox and they all looked fine, so I will post again after further disassembly. This is a 10520 Gold Series, 21".

The weather finally warmed up today so I spent some time with the Lawn Boy. Opened the gearbox to examine the shift keys and bearings. Wow! There has to be a trick for removing the keys, and I hope it doesn't entail complete disassembly of a drive wheel! Does anyone know how to remove the keys and/or remove the drive shaft gears? I cleaned the sliding tracks and even though everything looks good, I think I should remove the keys and check the matching key holes in each gear. Thanks!

Mystery Solved. Take a look at this picture of the worn bushing alongside a new one. This goes in the side of the gearbox and the output shaft/axle spins in it. It is mainly lubricated by a felt washer on the axle, also, a tight drive cable will accelerate wear to this part. This little nugget costs $17, but it did it's job well for 22 seasons, sacrificing itself to save wear on the output shaft. When it got to have 1/8 inch play, the gears had trouble staying in contact.

Here is a link that might be useful: {{gwi:126967}}

Excellent job!