Taking just the differential to a garage isn't going to be any cheaper. I don't have experience but have watched gears being installed and it's just as easy to have the whole car on a lift while doing it. I had a shop do mine years ago for about 120 bucks in labor costs.
That is really, really reeeeally cheap. Assuming most shops charge between 50-60 bucks an hour, thats assuming they did it in about 2 hours. If they do good work, I'd keep going back to them if I were you, because that is a killer deal. My boss just had new gears installed in the back of his Firebird, and he payed 1200$ total. Granted, a 5.0 has a pretty basic rear end, but again, 120 is really a good deal.
I did my gears myself without any prior experience, but I've had experience as a machinist, so I was familiar with using the calipers and dial indicator (which are essential tools for this job), as well as setting the backlash, etc. Rebuilding a rear end is somewhat a finesse operation, so if you've never had experience with precision tools, or don't have access to a friend who has done it before and is willing to walk you through it, I would recommend you don't try it yourself.
If you do, some of the tools you will need:
Dial (or vernier, if you're old school) calipers
Magnetic base dial indicator
Impact wrench capable of 200-250 ft-lb, IIRC (we got away with using an electric one)
A large torque wrench (I think I used my 25-150 ft-lb unit)
A BFH and a 2x4. Seriously. You'll most likely have to do some pounding.
A hydraulic press, or a machine shop to press the bearing off the pinion and maybe the ring off the diff.
Tall jack stands. The higher you get the car, the better.
Obviously, normal hand tools.
Supplies:
Full rebuild kit, I recommend the ones with all the bearings (replace your axle bearings while you are in there).
An extra crush sleeve or two. It took me two tries to crush the sleeve and get a good feeling pinion pre-load. These bad boys are one-use only, DO NOT REUSE. Once they are crushed, they are done.
Lube and friction modifier.
Lock-tite. I think you need the red stuff, but I can't remember for sure.
Some beer, if you are old enough. It helps ease the edge when you can't get the damn side shims in place.
MOST IMPORTANT: A good reference, or two, or three. The more research you do going into this, the better your results will be. Yukon Gear has a gear swap article on their website. Check it out.
Wow, that was way too much typing. Anyway, good luck.
