There is a very good setup guide on the internet by "Elvo" - http://home.scarlet.be/~be067749/58/
Fundamentally, of the parts you mention...
Shock springs - harder is more responsive, less ultimate grip, possibly worse over the bumps (rarely an issue in touring). Most touring cars are designed to use the same rate all round, or one rate stiffer on the front.
Damping - heavier damping will slow down the cars responses, and possible make it worse over the bumps. Damping should be matched to the springs, running one end of the car heavily sprung and lightly damped compared to the other is going to cause (or hide) problems.
Diffs - basic setting on a 4wd car is to have the front diff tighter than the rear. The rear diff is usually set quite free as long as you don't get the sense of it spinning up the inside wheel through the corners (unlikely in touring). A stiffer front diff setting will make the car understeer into the corner and through the middle, but will also make it pull more aggressively out of the corner. Like everything in car setup, it is about BALANCE.
There is no substitute for testing, and trusting your own reaction to the cars performance. And don't forget that these setup changes are meaningless if the car has not been built and maintained properly.