just to be clear are you using this guide: http://androidforums.com/evo-4g-all-things-root/655895-complete-unrooting-guide-evo-4g.html?
what method and what step are you on?
Yep, that's the one. I'm using "method 2- to fully unroot." Since my phone somehow changed to locked and hboot 2.18, I'm on step 2- installing the stock rom and recovery. I renamed the PC36IMG file and put it on the sdcard. When I load into bootloader it scans through some files but doesn't ask me to do anything. I tried going into recovery and loading the zip file from the card, but it cancels the install with an error.