With custom manufacturing and a little welding you can keep the brushguard on after a bodylift. By extending the frame mount points equal to the amount of the lift you can keep it on.
The brushguard wont be as strong as original and may have a little more vibration while driving, this will probably only be noticeable if you have lights mounted on it.
I did one for a 99 Tahoe a few weeks back that had a 2" bodylift, it took about 2 hours total to do, about 30 minutes of that was welding. A plasma cutter is almost a neccessity to complete the job.