I assume you have never modified your board... Let me explain a bit about doing that. If you already know that, you can skip directly to the directions of course.
UBB is a board which is built on Perl, and all its code may be viewable and modified by its owners. So if you want to change just about anything you want in your board, you can simply open a file, and modify it.
In order to have all the "http://gringos.com.lhost7.atlantic.net/gringotalk/ubb/" instances in the post bodies become "http://gringos.com/ubb/", do the following:
Open Notepad, and go to your UBB's cgi-bin directory. There you'll find a file called "ubb_lib_posting.cgi." Open it.
Find (with the Notepad find function):
$post_body = $thisline;
Right below that, copy and paste the following line:
$post_body =~ s/http://gringos.com.lhost7.atlantic.net/gringotalk/ubb//http://gringos.com/ubb//isg;
Then go to your UBB Control Panel, and clear your board cache.
If it still doesn't work, please provide a test user/pass to access your board.