Probably because it really varies by what shops charge for their labor rate. I think you should expect to pay $300+, and thats pretty low.
Just call around and look for a shop that has a good reputation in setting up gears. Or an off-road/4x4 specialty shop.
This is important cause if they fail to set the bearing preload and backlash properly... your rear end will destroy itself.
I was quoted $375 just for a more simple carrier swap. I shopped around and got it done for $150, at my friends uncles shop which had a good reputation with the 4x4 scene.
of course, add in more to the cost for parts and supplies.
afterthought*... Is that a 12 bolt rear? I thought those had 4.56 gears. Shouldnt that be good enough to run 33's? Or maybe you can just buy a 3rd with the gearing you want and simply just swap it in? Youd have to do the front as well, so more $$.
Question is what gears are you going to try and install?
You have a Corporate Isuzu 12 bolt rear end and a Isuzu 10 bolt front end. Your choices on gearing are limited.
Your truck came with 4.56 gears and 245/70/16 tires from the factory. To keep the same overall ratio you'd need to switch to 5.13 or 5.29, neither of which are available for either your front of rear axle.
5.38's used to be available aftermarket, but the 10 bolt gears are gone, unless you know where a set is. Isuzu made 4.77's that they installed in the RS Troopers, but you will have a very difficult time finding a set. Not to say they aren't out there, just pretty rare.
Well... the short answer is "I don't know"... This is a Trooper I used to own that I am probably going to buy back in the next week or so. I used to know what gears it had, but I really can't remember. It has been 3 years. It is going to need new tires, so I figured that I would get what I wanted right out of the box instead of putting tires on and then in 6 months wishing I had put something else on. I believe that it now has 265/75/16 on it, which is larger than the ones I took off when it came time to replace them... again, that was long ago. If memory serves me correctly, it has a posi rear end. I really can't remember.
For the last 4 days, I have done nothing but eat, sleep and crap Trooper. I have been looking back at the pictures I have of it and trying to make some sort of plans for when I get it back. I might go look at it and decide that it is not in as good of condition as it was when I sold it and pass on it... but sitting here doing nothing was driving me crazy, so I have been here reading and re-familiarizing myself with the Trooper platform.