There is a lot to consider when changing gears if you want to end up with a satisfactory result. A lot depends on deciding just what you plan on using the truck for. ie. Off Road only? A combination of On/Off Road? On road only? easy trails. medium tough trails, ********* trails, rock climbing/crawling,etc. Each comes with it's own gearing requirement to get the best overall performance. Big tires and lower gearing can put a lot of stress on the drive train and unless you plan on beefing it up, you are going to experience some bad moments.
ie. driveshafts, U-joints, axles, anti-axlewrap bars, etc.
If all you are going to do is put a light load on the drivetrain, you will probably get away with just changing gears. However if you are going to be putting stress on it, you are going to be snaping things unless you beef up and even then you may still snap something.
http://www.ringpinion.com/default.aspx
http://www.differentials.com/
I would suggest that you contact one or more of these sites and give them your specifications and get their expert opinion.
There are a lot more sites, but you need to do your research and decide what is best for your purposes.
Good luck,
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