Chevrolet Repair: no 4-wheel drive, chevy silverado 4x4, chevy silverado

Question
i just bought a 99 chevy silverado 4x4 with the 5.3 liter engine from my cousin.  recently the 4 wheel drive apparently quit engaging. he and a friend put it up on blocks and it tried to engage...making a clicking sound like the gears were not fully engaging. they reportedly replaced a black box solenoid thing but this did not help.  any thoughts on what could be wrong or do i need more specific info...like the name of the replaced part.  thanks for any help

Answer
the name of the part is an encoder, or shift motor. ok, i just came across this. first thing, you have to get all four wheels off the ground. now, have someone get in the vehicle. have them start the vehicle. next have them put it in 4 wheel low while you are down under listening. if you hear the shift motor engaging the four wheel drive, thats good. next, look at the drive shaft going from the transfer case to the front diff. here is where you find out what is going on. if that shaft is spinning, and the front wheels arent spinning, you have a front diff problem. there is an actuator on the diff that moves and locks up the diff. pull that out with the engine off, key on, it screws out. now, have someone put the vehicle in four wheel drive, if the actuator works, the problem is internal, and you are probably better getting a salvage yard one and putting it in. if the shaft isnt spinning, first, check for the proper fluid level in the transfer case. if thats ok, and you know the shift motor is installed correctly, you have a clutch problem in there. but make sure the shift motor is installed correctly. if the end of the motor isnt lined up with the notch on the transfer case, it will spin, but not engage the four wheel drive. any more questions, please ask