Front wheels not engaging (fully) when switching to 4WD

[Hassan Artail]

Few days ago, I put the 4WD shifter into 4WL of my 1998 Pajero (3.5L Sport), and user the front wheel indication lights would start flashing before going solid, but they did not, and so, I put it back into 2WD.
The next morning, as I was starting to drive it, while making a turn, a strong popping noise came out of the front, twice (about 10 meters apart), as if the gears of the front wheels got disengaged forcefully. Other than that, the Pajero has been driving fine, although I have not tried to engage the 4WD - out of fear I may damage something.

What do you think happended, and what do you recommend?