Still being written so there will be blanks!
This is the biggest discussion in offroading circles. Basically no matter what else you do to your vehicle if the suspension is not set up correctly then you'll not go nearly as far as you otherwise might. High priced toys or crazy lifts are not always necessary, I've bought some and found most of them lacking so here I'll try to outline my thought on the subject and how that was reflected in reality.
I won't waste time by explaining how it all works, nor on the difference between leafs and coils. Body lifts are very different and simply raise the body off the chassis to give room for bigger tyres. Looks good but can make the car very top heavy. The only one I've seen went on it's side first time out although there is more to it than that. I'm still thinking about it.
The [very] basics
By the time we get our mitts on the SJ the springs are flat, the bushes are rotten and the shackles seized. The U bolts have stretched and your car is both hellishly rough, bad in the mud (relatively speaking, we're not talking Landies here!) and possibly a death trap the an MOT tester will ignore, although he will spot the knackered shock absorbers (see below)
Bushes
These are the rubber bits that isolate the metal of the shackles & springs from the metal of the chassis. They allow for slight movements and should be replaced if they get too hard or rotten. Most serious off roaders fit polyurethane ones that are much tougher. Think of them as suspension for your suspension components.
Shackle lift
Springs are fixed at the inboard end and the other end must move forwards and backwards as the spring compresses and extends The shackles allow this movement. Fit longer shackles and the spring s 'pushed' down and the car moves up. Hey presto, lifted suspension. The amount of lift is about 70% of the length of the shackle over the standard one, so a 2" longer shackle will give an overall lift of about 1.5". If they are too long they may twist and damage everything else. Most long shackles are made of heavier gauge metal and are braced. You MUST fit the same ones on both sides of an axle - do not fit 1 longer one to raise a droopy side, it will ruin your steering and handling and could kill. I kid not
Spring lift - standard rate
You can get standard rate springs that have been made with more of a bend in them to give a 1" or 2" lift. Don't waste your money. They'll flatten out as they don't have the strength to hold the lift, especially if you're off roading.
Spring lift - uprated or heavy duty
Now we're talking. Available with standard height, 1", 2" or even 3" they're built to take it. The ride will be harsher but they do bed in and give you that all important ride height to get over obstacles as well as articulation. When added to a shackle lift 4" is possible but anything more than 3" is possibly bad news for the driveshafts and steering angles.
Spring Over Axle (SOA, SPOA)
From the silly but normal in the US file. Normally the vehicle 'hangs' from the springs, but you can change things to make the springs sit on top of the axle giving an instant 4.5" lift. This will bugger up the steering angles so must be done in conjunction with a steering kit. Involves welding spring perches on top of the axle housing although bolt on versions are available.
Shackle reverse
Um, shackles inboard, springs fixed front at the front and back at the rear. Slightly more to it than that but makers of the kits claim better articulation and ride, not worth the cash in my book. Most kits look awful and limit the entry and exit angles. Better spent elsewhere.
Jeep YJ conversion (JP-Eater)
I'm just about to do this so I'll fill you in later
Coils
This is the easiest to put to bed. Very, very expensive and tricky and a whole discussion in itself. Very few can either be bothered or can afford it so not really worth writing up. If you care enough do it for me and I'll put it here.
Remember to extend your brake lines as well!
SHOCKING STUFF
More appropriately called Dampers, they inhibit the movement of the spring so you don't bounce like Tigger when you hit a bump, i.e. they dampen the movement. They operate in 2 directions, Compression as the wheel travels up and rebound as it moves back. Extension is when rebound becomes downward travel below normal.
Choice of shock is bound by its length and strength according to the amount of suspension lift you have and the comfort level you want or the type of off roading you do.
Lifted
Articulation
Comfort