620i 4wd driveshaft not engaging (not electrical)

The front driveshaft coming off the real transaxle is suppose to spin/engage all the time, then when the 4wd button is on it engages at the front diff, correct? If I put it in gear and turn the driveshaft by hand it will engage, but once I put it in reserve it won't engage again.

Button, relay, etc is all working and tested.

Sometimes I hear clicking when the wheel locker is engaged.

Please help!

Thanks