I had the same problem on a free mower I got. After I got it running, I tried it out and it would not pull backwards without dragging one or both back wheels.
I solved the problem by taking off the back wheels one at a time and cleaning off all the little pieces on the axle. On the lawn boy master parts viewer there is a diagram that shows all clips,springs,washers and what not that goes on the end of the axle.
https://lookup3.toro.com/partdex/back2lst.cfm?xCaller=LawnBoy&o=2&x=20838 Click on 200% to see the diagram bigger.
There was so much grease and crud packed in these parts that the rocking key wouldn't work with the other parts. I just cleaned off each little piece and put them back on it the correct order and gave the grease fitting a pump of grease and it worked like new again. The rear axle diagram is a big help in putting all parts back in the same order. Just lay each piece out in a row in the order you take them out. I took everything off down to part # 22 the pivot washer arm.
Hope this helps. Good luck.
PS, be careful taking off the two retaining clips, they have a tendency to fly across the garage.
Jerry