lawn mower drive problems

hi first post here, ive got a problem with my sandrigarden (lawnking i believe now) mower, the drive wheels at the back don't move independently of one another, so they both either go forward or both go back cant have one going one way and the other going different. So everytime i turn the mower it rips the grass up. i understand that the drive bar at back has pinions at the end that are supposed to let the wheels turn differently, im trying to get these off but they just wont budge. any ideas? or am i on the wrong track? thankyou in advance

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