borden_hcc1775

i have a 91 cheyenne 4x4. the transfer case works fine, when i put it in 4 wheel drive it turns the front driveshaft. however im still not getting power to my front wheels. when i put the transfer case in neutral the front driveshaft turns freely. in 2 wheel drive the front axelshafts turn so im assuming there are no auto-locking hubs. so the only place left for error would be in the front differential somwhere. but i dont know where it engages... any ideas where i might find the problem?