Nomadic,
It's refreshing to hear someone ask logistical questions with a realistic, sensible goal in mind. You stated that your stock setup has been more than adequate for the terrain you drive in. That's good information for making a suspension modification decision. I can answer a couple of your questions, provide some info, and give an opinion. I do not know your level of mechanical knowledge so I'll apologize in advance if my information is to basic.
First off, there are two commonly used terms for suspension modifications. (1) A Lift, which both the front and rear of the vehicle are lifted to a desired height. (2) A Leveling kit, which only the front of the vehicle is lifted a small amount to level the vehicle and eliminate the factory rake. Basic 4x4 101 = To gain ground clearance, taller tires are required to increase clearance between the axles and the ground. To accommodate taller tires, a lift is required to provide clearance between the tires and body parts. Normally suspension height is determined by the tire size that will be used. Once these two factors have been decided, suspension performance is then considered and there are numerous products and brands on the market. I'll assume you're going to stay with a reasonable tire size.
Shocks for your vehicle are manufactured in 2 inch, 2.5 inch diameters and with external reservoirs. The differences are increased oil capacities to help prevent the oil from overheating when the shock piston in undergoing extreme, fast, rapid movement. Note: external reservoirs provide no benefit on pavement.
Lifting the rear of your Cruiser is simple. A taller spring or a spacer however I do not recommend a rear spacer over 1 inch. Forgoing a spacer and using a taller spring is the preferred method. Note: your coil sprung rear axle is centered within the frame by a solid Panhard bar connected at two fixed points on the axle and frame. When lifting the rear suspension the axle will shift to the right. To re-center the rear axle an adjustable Panhard bar is required which allows adjustment to lengthen the bar, re-positioning the axle. Adjustable Panhard bars are reasonably inexpensive.
When purchasing springs you must consider weight. Most aftermarket taller rear springs have a weight rating higher than the stock springs primarily to accommodate heavier loads such as a steel bumper or heavy cargo. If you install a heavier weight rated spring in the rear of your Cruiser without any additional weight added to the vehicle you'll end up with a stiff ride regardless of your shock choice. Bilstein offers a rear spring advertised to produce .9 inches of lift with the same weight rating as factory springs.
The front of your Cruiser uses a Strut in which the spring is seated on the shock forming a single unit. Most aftermarket front suspensions for the FJ are height adjustable either by pre-positioning the spring perch on the shock or by adding preload to the spring by an adjustable collar. Front Struts are offered in 2 and 2.5 inch diameters shocks but be advised that most 2.5 inch systems come with larger outer diameter springs requiring the use of aftermarket Upper Control Arms and a front sway bar relocation kit to clear the larger springs.
Aftermarket Upper Control Arms are only required if; you use large outer diameter springs, or lift the vehicle to a height which puts an extreme angle on the factory upper ball joint, or the lift height makes it impossible to achieve proper Caster adjustment with stock UCA's. I know several guys using front Bilstein 5100's with stock springs, set at 2.5 inches of lift, stock UCA's and running 285/70's. They report no tire rubbing and acceptable wheel alignment.
The most valuable recommendation I can give you is to sit down with pen and paper and formulate a detailed build plan from A to Z. What size tires do you plan to run. Do you need a full lift or a leveling kit. Are you adding any additional weight to the vehicle. Take into consideration that you may in the future want to alter your original build so purchase products that allow for alterations. Case in point, if you later decide on adding aftermarket UCA's you need a wheel off-set of at least (Negative) -12 to prevent the tire sidewall from contacting the UCA's so purchase wheels with this in mind.
You don't have to spend a bucket of money to achieve a lifted, quality ride. My first build was a full Icon system which I regretted and later removed and sold.
Given the mileage of your vehicle you should replace the shocks at all 4 corners even if you decide on just a leveling kit. 1.5 inches of front lift will level your Cruiser. If you raise the front 2.5 inches you'll need 1 inch of lift in the rear to keep it level.
Brands that I would recommend you research are Bilstein, Old Man Emu, and also Fox. For upper control arms, SCP and JBA. Use the manufactures websites and Youtube for information.
I'm attaching photos of my current setup for visual reference.
Front = Bilstein 6112's set at 2.5 inches of lift. JBA UCA's. Sway bar relocation kit.
Rear = Bilstein Springs providing 1 inch of lift. Fox 2.0 Shocks. Adjustable Panhard bar.
Tires = 285/70 17's
I experimented with Bilstein 5100 shocks in the rear but found them to be a bit stiff. The Fox 2.0's combined with the Bilstein springs that have a weight rating of 180 pounds is the perfect setup for me. My current setup provide a very comfortable ride on and off road with good on-pavement handling characteristics.
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