Sounds like the middle drive gears are damaged. You can physically inspect them by removing the front drive yoke from the engine and looking inside the engine. Drain the engine oil first, remove the 3 front differential mounting bolts, diengage the front driveshaft, remove the 4 12mm head bolts holding the front drive into the cases, and remove. If the gears are damaged, you'll have to pull the engine and split the cases to repair it.
There have been quite a few threads on this, try searching for more info. Let me know if you need any parts or assistance!