In case someone doesn't know how to replace their build.prop with the tweaked one, which goes in your /system folder. What it does is trick your phone into thinking it's an xt883 so it will work with gsm in the US. Be aware that all the links/instructions here are found elsewhere, so you can find this info if you search. If you need more help, search, but ask anyway if you need it. Just be sure to say thanks ;) Best way to swap build.prop is to do the following:
1. Download Root Browser, free version from Android Market.
2. When you are going through the process of following the steps in the original post, you should download a .zip file which should contain the build.prop file that you need, along with the radio.img you should've already flashed using cmd prompt commands on your PC. If you can't find this file, just download the .zip again, unzip and you'll find it there.
3. Connect your phone to your PC via usb and move the build.prop file to your external sd card (or whichever one you feel comfy with lol), to a folder where you will remember it and access it later from Root Browser.
4. After moving the file to your sd card, simply go to the folder you placed it in, in Root Browser, then long-press the file and select 'move'. Then navigate to your /system folder which is found in the '/' or root folder. There should be a paste button on screen, or press menu to paste the tweaked build.prop file in /system. This will replace your original build.prop, so you might want to save a copy of it on your sd card, but honestly you don't need it. You can always recover your phone using the fast/easy unbrick tool also found in the dev forum, which will put your phone back to stock.
5. Once you have moved the tweaked build.prop you downloaded from xda, reboot your phone like normal. If the radio doesn't pick up a signal now, you should go to your settings to change to gsm.
*Before this next step*
Be sure that if you see a dialogue box pop up at any time saying "there are no available gsm networks, would you like to switch to global mode?", select 'cancel', because it will just change your phone to the previous settings, which is bad. Global mode doesn't allow gsm, so you don't want it to pick that mode. Do not select the check box, as when you lose signal at times, this box will need to pop up again and you'll need to hit cancel every time. This tells your phone to wait until it finds your correct gsm signal (I believe this is fixed by one of the build.prop edits suggested here in this thread, by making the phone think it is not a verizon global phone...it will pick up signal after every reboot).
6. In settings, go to wifi & networks>mobile networks>network mode, choose gsm/umts, then tap back button to go back to mobile networks>select network, tap automatic, then go back to mobile networks>operator selection, deselect the checkbox that says 'choose automatically', then tap 'search for networks', choose T-Mobile 2G (or whatever Operator you use (you'll only get edge on T-Mobile)), and go back to mobile networks>Access point names, and if everything else goes well, this should show T-Mobile already selected. If not, google search 'tmobile apn settings' and type in the correct info you find. I can provide the info to type in if it still doesn't work, but it should work with anything you find online. Just type it all in as shown in pretty much any Android related article you find online.
If you make it this far with no problems, your phone should work just fine. No, nothing done here is very hard, it's just a lengthy process the first time.
There are more build.prop tweaks you can perform by using the text editor built into Root Browser, which works very well. You can find these tweaks in the same forum as the original tutorial, as a comment from someone. Be sure to be very precise any time you mess with build.prop, as your phone needs this file when it boots up, and screwing up the wrong line will brick your phone. You can always force it to recover using the fastboot (fast/easy) unbrick method I mentioned earlier, so have fun. Also, Root Browser will usually force close after tweaking build.prop, and you need to reboot to make sure you see the full effects of your changes.
Sorry this is so long, it was a PM to another member who needed it spelled out. Hopefully this will prevent others from having the same issues.
OP, this would go great in your guide, or maybe a link to it for noobs. I already PMd you about it, but I made some more edits here :)
Droid 3 XT862 back on Verizon now, using custom build.prop. Can't seem to get ICS to flash to my Safe system, ugh!!!