I can only comment as an RRC and D2 owner but will start by 100% endorsing what Philip A said - if you only buy 1 diff lock, it goes at the back. A quick home test for the viscous centre diff is to jack up one corner (just a little) and try to drive off - if it is working normally it will lock almost instantaneously and the 2 wheels on the ground will drive you off the jack. If it is buggered, the wheel in the air will spin. I reckon my viscous centre diff locks up off road damn near as good as my old FJ55 (which had no centre diff). The traction control on the D2 is really impressive (when it works) but not nearly as good as a locked or viscous centre diff because it can only work after a wheel starts to slip; and even then it only guarantees you 1 wheel drive (if you have no CDL).