4x4 hub clicking
 

I have clicking at the auto locking hub on the left side and 4x4 won't engage.

1994 Mazda b4000, same as a Ranger.

I took the hub off and cleaned if out. everything look in very good shape.
No signs of wear.

I reassembled and the 4x4 clicked a couple of times then started working fine.

An hour later it was clicking again and would not enbage.

The truck has 123k miles but has hardly evey been in 4 wheel drive.

Could it be the hubs are bad? or something else?

Thanks in advance for any help.

JoeV