If you increase the size of your tires, you will have to regear it to keep it in the optimum RPM range. Increasing the size of the tires causes the RPMs to be lower at a given speed. This also could make the truck a power DOG if you don't increase the gear ratio in order to keep the engine running in it's optimum RPM range. If you look at Jeff's Bronco Graveyard, 80 and Up catalog you will find a chart to aid you in selecting gears depending on how you want to use your truck.
Definately YES to the question about regearing both front and rear. If you don't the tires will not be turning at the same rate when you go into 4x4 and you will have a nasty surprise.
You probably can find a comprimise gear set that will work fairly well when switching tire size as long as you don't change tire sizes too drasticly. Other wise, you will have to satisfy yourself with a tire size and tread that you can live with under both conditions.
Good luck on your selection of tires and gears.
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