Well, don't bother installing any lift without at least adjustable upper control arms. You won't be able to adjust pinion angle if you do that, which will be paramount. I know another guy was having a similar issue recently because he bought the RC 4" lift and was getting all sorts of vibrations, but couldn't adjust the pinion angle because they don't give you adjustable control arms with it.
So that being said, I wouldn't put that spacer on unless you can adjust pinion angles (though the front rarely does need to be adjusted, so you might actually be fine). I think it would be fine though with the spacer.
Though the proper way to do this is with a quality set of springs that are "heavy duty" like the Currie's or OMEs, where they are meant to compensate for heavy bumpers, winches, etc., and won't sag in the front or rear.