Things to observe the next time you are under the truck.
Follow the power train back to the rear wheels. Start at the engine. You will see in this order. Engine, transmission, transfer case, prop shaft, differential and finally the rear axle.
There are shafts, spindles, bearings, gears in the transfer case spinning to apply power to the prop shaft when in 2wd.
Now, look at the transfer case. You will see a shaft going forward to the front. The order from Xfer case forward is drive shaft, front differential, axles, front wheels.
The drive shaft only has power when the xfer case has been engaged. The power is transferred to the drive shaft by a chain. One end of the chain is on the spindle applying power to the rear. So, the chain is always moving (ie rotating).
So, considering the miles you have......the chain may well be worn. Simply by the millions of rotations.
To my knowledge, there is no way to detect the amount of chain wear short of dropping the Xfer case to crack it open.
There maybe a Junior College offering car maintenance classes. It could be a good way to shorten your learning curve.