4 Wheel Drive/SUVs: 2003 GMC 2500 HD 4wd, posi traction rear end, clutch plates

Question
4wd will not engage. replace actuator today

Answer
Wes,

I am not that familiar with this model.  From what I understand, there are a couple of issues with this vehicle.  If it has the full time transfer case, it is very important to maintain the fluids as it has clutch plates in it (very similar to a posi-traction rear end) and they can start slipping without the proper maintenance.
The other problem is the front differential has to lock in.  It has an electrical motor that locks up the axles for the front wheel drive.  If there is low or no voltage going to this unit, the front end will not work.
I would suggest that you purchase a repair manual at your local auto parts store.  They cost around $20 and are a great source when trouble shooting or repair of your vehicle.

Carl