UPDATE (3/19/2012): The most recent D2G update (4.5.629) currently has no known root exploits, and cannot be downgraded via SBF. Only D2Gs on Froyo (2.2) or Gingerbread (2.3) prior to the most recent update have the ability to be rooted.
METHOD 1 (compatible with Android 2.3 Gingerbread, prior to the 4.5.629 update) (courtesy of Pete Souza)
1. Follow the instructions here.
METHOD 2 (compatible only with Android 2.2 Froyo) (Courtesy of RyanZA on xdadevelopers)
1. Download and extract the z4root.zip file and copy the .apk directly to /sdcard.
2. Go to Settings -> Applications and make sure "Unknown sources - Allow installation of non-Market applications" is checked. Then click "Development" on the same page and make sure "USB debugging" is checked.
3. Open the "Files" app, navigate to the z4root location and install.
4. Launch z4root and select the Permanent Root option. Give it a minute to do its thing and reboot. If it spends more than two minutes on the message "'Running exploit in order to obtain root access" then you'll want to restart the app; this happens sometimes. Just exit the app by pressing the Home key, kill the app via a task killer (or Settings -> Applications -> Manage Applications -> z4root -> Force stop) and run z4root again. Voila, you're rooted! (You can also use z4root to unroot.)
5. Wireless Tether is a good first app to download if you're rooted. Download the most recent version of Wireless Tether (v2.0.7 as of 10/11/11) to test if you're rooted properly here
CUSTOM RECOVERY TUTORIAL (Courtesy of Koushik Dutta aka Koush)
1. Download the Droid2Bootstrap.zip file at the bottom of this post, extract the .apk file and place it on /sdcard.
2. Go to the "Files" app, locate the Droid2Bootstrap.apk on /sdcard, install it, and open it. (Select "allow" when asked whether to grant the app superuser permissions)
3. Click "Bootstrap Recovery" and wait for the "success" notification. Then click "Reboot Recovery" and you're done! This will boot you into recovery where you can make nandroid backups, install from .zips, etc (do a search on the forums for more about custom recovery if you aren't familiar with it)
4. DO NOT delete the Droid 2 Bootstrap app once you have completed step 2, as this is the only surefire way to access the custom recovery. Holding "X" + "Power" will take you into the normal recovery.
SIM UNLOCK TUTORIAL
1. Make sure your phone line has been active and had no late payment history for at least six months. If it hasn't, VZW will not unlock your SIM.
2. Call VZW Global Support at 1-800-711-8300 and ask the representative for your phone's SIM unlock code.
3. To test it, you will need a non-VZW SIM. Once you insert it and power your D2 Global back on, it will ask for the unlock code.
NOTE: It appears that Verizon has put a carrier lock on the device for US GSM carriers. You will not be able to use this device on AT&T or T-Mobile in its current state, even if SIM unlocked, unless you send your phone in to DroidBandUnlock. They charge $100 for the service and it only works on certain software versions. Read more here.