Sears Front wheel Drive Lawn Mower- Wheels Froze Up

I have a Sears front wheel drive lawn mower, 21 inch 6.75HP, model # 917376551, that is about 6 years old. Today while mowing, when I engaged the front wheels by squeezing the lever on the handle, I heard something “POP” and then the wheels froze up. The drive wheels had been making a “slipping- as if a gear inside wheels was slipping and then re-engaging” noise periodically for the past few months. I took the plastic cover off the drive assembly area and the cable from the handle lever is not broke, it stills seems to move the lever that is attached to what appears to be a gear box. Both wheels are frozen and will not turn by hand. I read some of the previous posts in the forum, but still getting educated on mower repair. Any ideas on further troubleshooting to see 1) what problem might be 2) is it something I could repair(reasonably handy) 3) not cost effective to repair 4) or any other things I failed to think of ? thanks in advance for your help.
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