Ahh... Sorry, here:
On the Eris:
Download
Root Explorer or a similar manager. I have Root Explorer so I'll write the steps for that, but you could try
a free one like this. Root explorer is nice, though.
Open Root Explorer and grant it Superuser permissions.
Navigate to the Data folder, then to App.
Scroll through the list and look for the file
com.google.android.apps.maps-1.apk or a similar name.
Long press on the file and hit "Copy"
Navigate to the top level by pressing the ".." folder at the top of the folder list until you can't go any further.
Open the sdcard folder
Press the Paste button at the bottom.
Now your SD card has the Maps application on it, you can move it to your computer, and then onto the SD card of your Droid X. Once you have put it on the SD card of the Droid X:
On the Droid X:
Uninstall the Maps application
Open your Files application (this came with your phone)
Select Phone Files
Press once on the Maps application. Then select Install.