Changed battery in John Deere X320. Mower will not go forward or backwards now. The parking break if off, the manual rod in back which you pull out to manuallty push mower is not out... all the way in. Fuses are all good. The PTO works; blades spin. Any ideas or suggestions are appreciated. Thank you.

Battery and not moving are not realated issues. Have you inspected the drive belt off the main pulley back to the trans? It must be inplace a tight when the clutch/brake is released

I have checked the belt. And after talking with a mechanic, he said the belt should actually be turning when the mower is running... it is not. So, he thought it may be transmission related. Does that sound possible?

If the engine is running and the belt is not turning it's a belt tension issue, with the clutch out.