You are crazy for off-roading in West Java in the rainy season without MTs. I mountain bike there, but I stop during the rainy season due to the abundance of mud, that greasy red clay mud that is slippery like no other. Even my knobby mountain bike tires get clogged.
After driving down the river-roads, I would immediatly change out the fluid in your differentials, transfer case, and replace the u-joints (unless they're greasable).
Also, if you haven't already, do the rear differential breather extension mod-
http://www.thenewx.org/forum/showthread.php?t=54
Nice. Do you know where I can get the ARB rear locker for the Navara here in Jakarta?