I have noticed a clunking metal on metal sound when accelerating or breaking(even lightly),
but only when driving in 2WD with front hubs set to free.
I have also noticed that the driver side hub is quite hard to tun from locked to free in comparison to the passenger. (Can still be turned by hand.)
The truck is a 1982 LN46
What could be the issue?
Can i drive with the transfer in 2WD and front hubs locked until i manage to fix the issue?

 

Checking the hubs is pretty simple. Set them to free position and remove the 6 bolts on the dial face. It should pull straight out with the inner hub gear attached.
I had one that wouldn't unlock after a trip a while back. Turned out the spring broke. I just removed the spring and inner gear, bolted the dial back on and drove home.
A new set of hubs definitely fixes the "hard to turn" problem. Usually the O ring around the dial swells, breaks down, and just gets full of crud and doesn't want to turn anymore.
Let us know what you find.