Don't worry about it -- the 3 Amigos are very common. Many people drive thousands of miles with no problem with the 3 Amigos lit. In fact you'll find quite a few 4x4ers that intentionally disable the ABS system because in many cases on the trail you stop more assuredly with 4 wheels locked than chattering away on ABS. On my first Rover, a '96 D1, I pulled the ABS fuses because I hated the way the truck braked on gravel w/ABS.
I'm not suggesting that you shouldn't look further and get a diagnoses, only that you're dealing with a routine issue.
On your D1 the ABS was pathetic and probably needed to be disconnected off road (I do the same on my '98), but on his 2004 it's pretty useful. With the 3 Amigos on he's lost not only the party trick (hill descent control) but traction control as well. It's something that should be dealt with, and can indicate anything from a shuttle valve failure to a bad hub.