4-wheel drive issues. 2002 blazer. Intermittent kathunk while driving
 

I am having some issues with the 4 wheel drive on my 2002 chevy blazer. While using 4x4, I am getting some harsh noises while driving. I first noticed it when I accidentally left it in 4 wheel drive while driving on dry pavement.
The 4 wheel drive works, and I don't seem to notice the problem while driving on very icy roads, but if I am driving on partially icy, snowy roads that have some dry patches and some grip, I get a harsh thunk from time to time. the noise seems to be from the front end but who can really tell. The vehicle also seams to engage in 4x4 much harsher when shifting from 2-wheel drive to 4x4.

Its a 3 button transfer case. I inspected the vacuum lines and vacuum actuator under the battery. the actuator and cable that engages the front diff seems fine. I can pull the the cable by hand.
About 5 years ago, I had the transfer case worked on. I replaced the vacuum switch on the transfer case. The auto repair shop ended up opening the transfer case and replace the shift fork inside

Any guessed out there on what the problem can be now?