Can you post a link? I've looked on RootzWiki and what I've found is all saying that there is no root method for .621 (which is what this thread is about).
Update: found the thread and possible root method being reported but the Dropbox the sbf file is being hosted on is locked due to high amount of traffic.
The method basically involved sbf-ing from .621 to a Milestone .604 sbf file and running a manual exploit to gain root and then from there you can install the Droid 2 Bootstrap and then flash any ROM you want on your Droid X.
A user named bukolov1 from the Ukraine is the one that originally posted this method.
I suggest anyone who has SBF'd to 621 or done the OTA and lost root, to check out this thread on RootzWiki. It isn't root for 621, but its a viable workaround. What was found was that an SBF for the milestone X (sbf 604) could be flashed over 621, and then zergrushv4 could be used to root that. Now to do this you MUST use SBF_flash on Linux, otherwise you will flash your baseband and your data will not work (this can be corrected by a call to VZW asking them to reprovision your data). Once on this 604 your phone will report as a Milestone, but you can install BSR/CWM and flash over to a *GB* rom (froyo roms will not work, IIRC) and after that it'll report as DX (this can also be achieved using a build.prop edit, but who really wants stock? If you do just want stock and you do this build.prop edit, you will need to freeze/remove the updater or else it'll freak and try to update and bug you incessantly.) Hope this helps! I've got a dx2 personally but I've been following this thread hoping to glean some information for when our next update drops (which was supposed to happen like a week ago... -__-)