Snap-On & other Co.'s make a tool to knock those out. I've known front end guys that have made their own, that actually work better. For a guy that knows what he's doin', it's a not a small job, but it's not a '10' on the difficulty scale. More like a 6.5. It's worth it. With those outa the way, now you have more adjustment to have the front end set up to the way you drive. You can have them set so the tires ride flat, in a straight line, instead of riding on the outer edges of the tire, as they are now. Your tire will wear more evenly, some of the 'twitchiness' will be gone, and, depending on how you drive, maybe better mileage. My blue truck is set up that way. The black one is set up so that the outer tire is flat during cornering. Lotsa camber. Hard on tires. Tiring on the highway. A blast in the corners!
But those slugs gotta go. Part of owning a GMT400.
The upper control arm actually has a pre-cut outline of the piece that has to come out. Why they didn't do this from the factory is beyond me. The tool is just a mini hand sized press. Put it on, hit it with an impact, boom, out. The job is getting to that point, then putting it back together.