If you're talking about changing the final drive (ring & pinion), it's pretty easy. The trucks have a removable 3rd member that unbolts, and you set the R&P up outside of the truck.
http://www.off-road.com/trucks4x4/ar....jsp?id=274535

 

regearing these trucks is one of the easiest diffs to work with because they are third member style, the only repeated pressing you will have to do is pressing the pinion bearing on and off to change the depth shim stack.

as for swapping search on here for 3.4 (5VZ-FE) swap, if you want more power and still decent economy thats your best bet, you can find an R150 transmission from a 3.0L truck for next to nothing and use that aswell to save $$.

 

These gentlemen above say gear changes are easy, and it can be if you have access to a large press, torque wrench, dial depth gauge, and have the technical know-how to perform such surgery. My advice would be to thoroughly check out www.gearinstalls.com. ZUK does nothing but TOY diffs, rebuilds and installs of gears, bearings, and lockers. That site still doesn't quite do it justice as to how much time it takes to do what he does. If you want to tackle the job yourself, that is the site to reference. The hardest part is setting the pattern for wear, because you have to really know what you are looking for. Otherwise, gears go boom quickly.

My strongest advice if you want to save future work.....if you regear, put a locker in NOW, save you alot of work down the road to install later.

 

My 20R pushes my truck along pretty good with 35's and stock gears (4.10's). Im running a Weber 32/36, A header and 2.25" exhaust, Probably close to 130hp/160ft.lbs at the crank timed and tuned. Only time its "slow" is on hills at hwy speeds, Im re-gearing soon because my low range is too fast for trails.

 

If I re-gear I would try to find someone local to do it for me. I have the patience and if I have a DIY for it I could do it but I don't have the means to do it(not enough tools or space)