Shotgun...the rear shock is REALLY easy to rebuild and clean up.
Just get a big work table and start disassembling everything in order like you would a gun, making sure the reservoir bladder has no pressure.
Aluminum body shocks get sludge built up in them from the powdered aluminum wearing off the sides, and this is generally what causes poor performance.
There's also the matter of inadequate fluid in the shock, as well as inadequate reservoir nitrogen pressure.
Buy a set of seals for the rebuild, don't reuse the old ones (you can probably find them on bike bandit or similar online stores).
When you have everything disassembled, in order, clean all the sludge off the components and inside the shock, blow air through the remote reservoir hose to make sure it's clean and doesn't have a blockage in there.
Inspect the reservoir bladder for any tears or damage.
Put the reservoir back together with the bladder, without filling it with fluid, and put a few PSI of air in there to see if the bladder is leaking.
If the bladder is leaking, you'll emulsify the fluid in the shock with nitrogen and it'll reduce performance considerably.
For fluid, you can go with a specialty shock fluid, or you can go cheap with ATF/hydraulic fluid.
I recommend ATF/hydraulic oil because of how I assemble the shock.
The key to any shock rebuild is making sure you have no air inside the shock, not even tiny little bubbles.
I use a large container that I can completely submerge the shock and reservoir in, and assemble in the fluid preventing any air from getting in the shock.
Once you have the shock assembled without the reservoir attached, while keeping it in the fluid, compress and extend the shock several times to push out any air bubbles.
You probably won't have any if you made sure to assemble it in the fluid, but I have gotten a few tiny little bubbles at the end of the compression stroke a time or two.
Once you're sure that you've gotten all of the air out, leave it in the fluid and move on to the reservoir.
With the nitrogen bladder empty, remove the cap and bladder, then plug the hose.
Fill with fluid and reassemble.
Submerge the hose in the fluid and unplug the hose, then use shop air to pressure up on the bladder.
You don't want to fully pressurize the bladder, just enough to push the excess air from the line out.
A bicycle pump is good for this, since you can control how much pressure you add easier.
If the container is deep enough, try to angle the hose up, still submerged, before you pressure up so it'll get as much air out as possible.
I've had to deflate and inflate the bladder a time or two to get that last bit of air out.
Once you're satisfied that all the air is removed from the reservoir, install the hose on the shock and tighten.
Congrats, you've now assembled the shock without any air inside!
The last step is simple enough...you need to be sure you haven't overfilled the shock with fluid.
Be sure there's no air pressure in the bladder, first off.
Clamp the shaft end in a vise (with the end on the shaft) and compress the shock fully.
If you can't get it to fully compress, you've overfilled the shock.
Loosen the hose while compressing the shock and it should slowly leak out of the connection until it's fully compressed.
Be ready with a wrench to tighten it up as soon as you reach full compression because otherwise you might suck some air in.
Extend and compress the shock a few times to be certain there's no air trapped in there, and that it will fully compress and extend, then clean it up.
Install your spring on the shock and bolt it up to the quad.
Pressure up on the reservoir and you're done.
Not a good idea to pressure up on the shock and then try to install, because the pressure will push the shock out to full extension and make it harder to install.
You can still install the shock, it just takes a lot more effort.
Shop air isn't quite enough pressure, so you'll want to take it to a shop with nitrogen available.
I believe the standard pressure is somewhere around 165PSI.
I know it's not the easiest or fastest way to assemble a shock, but every time I've tried doing it the traditional way I wind up with air in the shock.
The normal way is to hold the shock vertically, fill with fluid, and assemble.
Once it's put together, you compress and extend the shock with the hose off to work the air out, filling with fluid as needed.
Fill the hose up and attach, then call it good.
I'm skipping around a bit and don't have any pictures for you, but it's actually really simple once you start taking it apart.
I rebuilt mine without any directions at all and I never felt like I was in over my head, just keep everything organized and you'll have no problems.