I really dont know all that much, so don't take anything i say without a grain of salt.
The one question you had that i can answer for sure is that you most defintely need to do both the front and the rear. If you do not do this, you would either a) only be able to use 2wd or b) your front and rear tires would spin at different rates. Probably not a good idea.
You didn't mention if you had a manual or an auto, but if you ONLY use it for trails, and low end is what your looking for... I dont see why you wouldn't to just shoot for the moon and go to 4.88 gearing. If anyone disagrees with me, listen to them. Like I said.. im a noob