My yardman self propelled rear wheel drive mower has stopped has started locking wheels(rear) when I try to pull it backwards. I have taken gearbox apart looking for chipped gears,didn't see anything but didn't wash out grease and replace, maybe should have, checked chain box, noticed lots of gray powder, either graphite or metal powder from axile wear maybe....Still locks up???

There is an excellent 3 part video that may help you to fix this problem. Please see the 2nd and 3rd videos:
https://youtu.be/Vk_tpjKlbjM
https://youtu.be/DdiewLxTQDc
Things you can try:
(1) Remove casing over rear wheel drive assembly and perform a test run to assure all parts are moving as expected
(2) Lubricate the shafts with lightweight motor oil where the shaft penetrates into the gear box of each wheel
(3) Remove each rear wheel, inspect gears for damage and free movement. Grease up each drive gear and the shaft.
(4) Inspect gears on each wheel for damage.
(5) If none of the above fixes the problem, try swapping out the front wheels to the rear wheels provided the front wheels can accommodate the gearing on the rear wheels
(6) If this doesn't work try replacing the small gear assembly on the right rear wheel
These steps are illustrated in the videos.
Good luck and hope this helps.

your drive will driver dogs are full of dirt you need to remove both wheels and clean the dirt out the dogs are spring loaded be ready for parts to spring out maybe depends on how much dirt

Leave aside all the puffery about 3-1 and such, you described a front wheel drive (pull) walk-behind (FWD), but are asking why the rear wheels won't propel. When you push down the hand grip the front wheels should spin free. Whan you lower from wheels by releasing pressure on hand grips, turning wheels lowered to ground should propel the mower along with its unpowered rear wheels now turning be ground contact. Reminder, never, ever lift rear drive wheels of any mower off the ground while walking behind with engine running:

1. You could be struck by blade propelled object.
2. You could pull mower and spinning blade backward onto your feet.
3. You could lose balance and be pulled forward in a fall.

Contemporary FWD mowers are recognized by a black chain housing on deck top leading from engine to left front of deck. RWD propelling chains are situated underneath deck so cannot be seen from above.

try pushing it forward just a little before pulling back hopew it helps

pull off what looks like hub caps and check the wheel clutches

The rear wheels are locked to the drive train. To pull or push the mower, you must pull out the release probably at the back of the mower, near the transmission.
Gary

The self propel system uses a type of racheting pinion drive that sometimes gets greased up and wont release like it should.

You would need to remove the wheels and pinions and look at the shaft ends for a key, a ball and a spring. This is the "pawl" that locks the wheels going forward and rachets when the wheels go backwards.

Clean up the parts of all the aluminum colored grease and reoil with something lighter. 3 in 1 would do nicely. Reassemble.

Now the next time you start going backwards and it jams...just push the mower forwards slightly without engaging the drive and see if the unit doesnt free itself.

Good luck and be careful!

turn it off,let the drive handle go,pull mower backwards.does it lock the back wheels?if you push it forward,can you hear a clicking noise from the wheels?you should do.lift mower,spin back wheel,should either lock,or click,depending which way you spin it.they have spring loaded pawls that drive the wheel.usually easy to fix.usually dirt,or too much grease.take wheels off,have a look

Typically self-propelled mowers are propelled using the front wheels, which only turn forward. Pulling the mower back while the front wheels are turning will damage the drive gears. It's best to raise the front wheels slightly off the ground before pulling back.

SOUNDS LIKE THE PINONS ARE ON BACKWARDS. THE KEY SHOULD SLIDE OFF THE PINIONS WHEN PULLED BACK AND LOCK WHEN ENGAGED. YOU HAVE THE OPPOSITE PROBLEM OR ITS BACKWARDS!