4x4 Binding...then freed up with pop

I was going through some wet slick terrain a few days ago and decided to put it into 4LO and lock the rear diff in an effort to reduce wheel spin and trail damage.

I noticed on this particular part the truck seemed like something was holding me back. Got out to check and couldn't find anything. Slowly crept forward and then the binding was gone with a pop...

Did a trail inspection and everything seemed to work fine. Didnt happen again the next 15 miles of trail and no adverse effects 200 highway miles back home. What do you guys think? Video linked

https://www.youtube.com/watch?v=d4MjRMxCQUs&feature=youtu.be

 

That's the same sound an auto locker makes with the teeth slip on each other after being in a bind. IDK what it is in your case.