4x4 partially engages

i have a 93 gmc sierra 5.7 i've replaced the 4x4 actuater with the updated one from GM. it works fine except that when its raining out ,it partially engages. it sound like a winding and grinding sound, and this is when the the 4x4 shift is still in 2 wheel postion. i can get it to stop grinding by shifting into 4 wheel and then back into 2 wheel . any help please reply hereor to snipped-for-privacy@cox.net thank you