For reference, you are not in 4 wheel drive if your hubs are locked, four wheel drive is only engaged when you put your little transfer lever into 4H or 4L and your yellow dash light should be on, this will engage auto hubs, but when you go back to 2 wheel drive with that lever the light will go out and you will be out of 4 wheel drive but hubs will still be locked until you reverse, keeping the hubs locked is ok, you are in preparation to use 4 wheel drive again it will not cause any damage just having the hubs locked, so when the conditions improve and you know you wont need 4 wheel drive much or again, you can reverse to unlock the hubs when it is safe to do so, you only need reverse a meter or two.
This is for the A series .
When you get stuck in mud with auto's on you tends to reverse to help get you out of the mud, that action will for a very short period unlock your auto hubs then relock them, this causes stress on the drive shafts, then when you engage a forward gear, again you tend to hit the gas, this again unlocks the hubs for a wheel turn or so and then it locks up, and this is a massive strain on the diffs and shafts and sooner or later something will give.
On manual locking hubs, they are permanently locked and you dont have the " snatch " effect on the drive shafts thus eliminating a huge strain on them.