The spring is a coil spring wrapped around the shaft under the vacuum chamber. The coil pushes the axle into low range. Vacuum overcomes the spring and pulls it into high range. I am assuming you have one hub off and that side on a jack stand. Try moving the shift lever which is on the drivers side down low facing the front of the truck. Move the lever down as far as it will go. Stick the axle into the housing until it is splined up. Turn the drive shaft with the transmission in neutral and see if the axle is now turning. The GMC maintenance manual does not address the subject very well. It assumes the user to have a general knowledge of 2 speeds. I can take a picture of a vacuum 2 speed and post it. That might help you figure out if you have parts missing. The cup is pressed into the hub, but it does not require a hydraulic press. I have a 3 prong tool to install and remove cups, but it can be done with a long punch. If you look from the brake drum side of the hub, you will see a small part of the cup protruding out. Tap it there on one side and another and it will come out. You should use a punch with a good square edge so it doesn't slip off. It will take a pretty good sized hammer. Do the reverse to reinstall the new cup. Be very careful not to slip off and scratch the bearing surface. You will be able to feel and hear when the cup has bottomed out.
