OK, diffs provide an advantage in that the drivetrain isn't locked into full time 4x4. If the front or rear wheels are bogged or stuck, all the available engine power is transferrred to the wheels that can spin. Plus they offer tuning options for dramatically different behaviour based on the weight of oil (actually it's not oil, it's silicone) you put in them. It's well worth having as an option part, especially if you want to race. If you do a search you will find a lot of info, that's probably why no-one's bothered answering this post until now. And also why I'm not going to bother explaining what different silicone weights do. It's a very often discussed subject around here...

As for getting the oil in/out, there's no trick to it, you open the diff, fill it 3/4 full, put the diff back together...getting thick silicone out is a nightmare, we all have different ways of doing it, but it's never easy.

Hope that helps...why don't you get a buggy instead, much faster, tougher, cooler...more raceable...