99 chevy 4x4 doesnt always engage
 

I got a buddy who has a 99 chevy silverado 4x4 with the 5.3. He just rebuilt his transfer case because his chain was stretched. Now when he goes to shift to 4wd, it will only engage every now and then. No noises are heard when it doesnt engage, but the light does come on. Any suggestions? He plug something in wrong?