I just got done fixing this very same problem. Removing wallpaper is the job from hell isn't it? I ended up putting a skim coat of joint compund on the whole wall and sanding down with a power sander (150 grit). It came out really nice but took some time to get completed. If you go this route, mix a little water in your joint compund to make it more workable, the job will do alot faster. I used joint compund rather than spackle cause it's more workable and seems easier to sand. Good luck