Rear wheel locked while riding. won't roll forward under power, with clutch in or in neutral but will roll backwards.

Kevin, if it is in neutral or if it is hydro-stat, you have pulled the pins in the back to unlock the axles. the brakes may be locked up. the last two mowers i got for some reason the brakes were locked up on them both. hope this helps.

sounds like shift lever adjustment problem
take it into a service center and have it properly diagnosed and a quote

bad bearings in the rear wheel

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

Broken selector claw spring

Ride Safe
Bike-Doc

i would remove the clutch assy and check all parts including rear bearings ! something is failing !

If your rear wheel is locked up in first and second gear with the engine running, this is not a good sign. Have you ever ran the bike without oil in the transmission? Every time I've seen this, the chain was thrown off the bike and it busted the cases. The oil leaked out of the cases and the transmission locks up. Generally, it locks in fifth gear but it could lock up in any or no gear.

Your problem sounds like something is wrong with the low gear-second gear "clutch gear". The clutch gear is gear that slides sideways on one of the transmission shafts to engage the proper gears. If the bike will roll when in neutral, these may be the only gears affected. If the bike will not roll when in neutral, your problem may be in countershaft. Either way, the transmission could be damaged.

Now, if you're talking about the rear wheel locking up if you try to push start the bike, this is because in low and second gear, there isn't enough mechanical advantage in the transmission to overcome the engine compression. Remember, you transferring torque through the transmission backwards when you try to push start the bike. Try starting the bike using third or fourth gear instead. Standing on the pegs and dropping onto the seat when you release the clutch helps.

I assume it's a manual transmission.
If the car will not roll backward or forward in neutral (with or without the clutch engaged) the transmission is most likely locked in two gears at the same time.
This may happen if gear selectors are excessively worn. If you are able to examine the gear selection levers external to the gearbox, ensure they are in the 'neutral' (no gear selected) position when the gear lever is in Neutral. If not in correct position, you may be able to manually reset the levers to correct neutral position and thereby unlock the gearbox.
In any case, if it's happened once, it will happen again, so get the gearbox and gear selection mechanism checked for wear or breakage.

Craftsman-Husky...same tractors-built in the same factory. I also had a problem with the hydrostatic transmission not providing power to the rear wheels. Here's what Sears told me:
(1) Check to make sure the drive belt is tight both under the engine-and the tranny.
(2) Air can get into the hydrostatic transmissions-especially during shipping (new mower) and storage.
So.....
Pull your freewheel pin out to disengage the tranny-then push the mower both forward and then backwards for 50' or so. Then push the freewheel pin back in and start the mower. If the transmission then works-drive forward-then backward-then forward repeating the cycle about 4 times. Drive about 50' in each direction. This should purge the air from the treansmission.