you do not need to be unrooted to install 181 build...

My RAZR is proof of that
just make sure your on rooted stock 173 build, all bloatware installed and
ABSOLUTELY NO BLOATWARE DELETED, no Bloatware frozen, no system file mod'd with ninjamorph/metamorph/setcpu/rom manager/font changers/adfree/etcetcetcetc...
open rootkeeper, make SU backup, exit rootkeeper, download/install 181 build, let the phone do its thing, phone reboots, successful install of 181 build, you'll be unrooted obviously, open rootkeeper, click RESTORE ROOT, bam... ... bam... and wait for...
BAM! rooted 181 build...
If your install failed, then you mod'd something during your rooted time on 173 build... you can agree with me or disagree with me..... but thats ALL I have to say about that...