From your description, something did let go in the transfer case if Park does not hold. The Park gear is the transmission output shaft. If it were transmission related the output shaft or splined connection to the transfer case would be broken. If you have manual transfer case, the fork may have broken allowing it to slip into Neutral position. The ESOF doesn't really have a Neutral position, but something similar could have happened. Sounds like the Park pawl is ratcheting until it engages due to no load on the drivetrain.