Yesterday I tried to put truck in 4low at boat ramp and it did not engage , and made loud knocking from TC . 4x4 light flashed . I moved switch back to 2hi and let truck roll about a foot and tried again. Knocked once and engaged. Any thoughts on what is going on? Oh, is 97 scab, 4.6, 128k miles with electric switch 4x4.

 

Bump..nobody has any input?

 

There is an electric clutch that is supposed to make two shafts get to the same speed then a coller slides in place to lock them togather.
No doubt you heard this assembly not fully engage.
Sometimes it takes a little fiddling to get them to engage.
A mild clunk most of the time is the vacuum motor changeing positions on the front of the case.