If it is a popped noise that sounds like a chain slipping, it isn't in 2wd, it is in part time 4wd and the drive chain in transfer case is slipping over the gears.

The chain should be replaced at 100k.

Is it a NP242 or NP231? The 242 have a problem with wearing a grove and making it difficult to get out of full time 4wd and into 2wd.

Put it back in full time and while going slow put the transmission in netural and then put the 4x4 selector to 2wd. I have to be going about 20 mph in mine but it works every time.

Once you have it out of 4x4 see if the noise has gone away.