I don't know if you can overthink this stuff but coming from a long engineering background of course I would say that
You could throw on a lift kit with beefy springs but it'll be a rough ride with compromised handling when it's not loaded up. There's also the front driveshaft angle and increased vehicle roof height to think about.
Here's my suggested approach to achieve this in a managed way without doing a HD lift kit.
- Before you do any/more mods get your vehicle weighed, including the front and rear axle weights. Clean out the vehicle but fill the fuel tank.
- Do any planned mods that will grossly impact weight, like winch, roof rack, bar work, etc.
- Load up the vehicle to your typical trip weight. Fill the fuel tank then weigh the vehicle again.
- Do the math for the total weight increase and front/rear axle loads (see below).
- Select and fit the new springs (also below).
- Add rear load assist airbags for towing to offset the hitch downforce (you know where to look).
Robert Pepper did a deep dive into 4WD payloads using the Grenadier as an example
here. Note that he is using Australian weight and load limits which might be different to other markets.
Spring Selection.
Look at the symbols on your current installed springs. Look up their kg weight rating in the table then select a new spring part number based on the weight gain and split. Check with your dealer what springs are available. This may still force you to go aftermarket but keep an eye on the spring length. The springs in the IA table are different load ratings to match the build weight based on the vehicle but they're not a lift spring or a HD spring.
Axle Loads.
Winch weight can reasonably be assigned to the front axle.
Rack weight and loads could be split 40% front axle and 60% rear but you need to think about what you carry on the rack and where it is located and adjust the splits accordingly. 40/60 could easily be 30/70. Weight acts downwards through the centre of gravity which shifts forwards during braking so if you must carry heavy loads on a roof rack (not recommended) then increase the weight assigned to the front.
Adding weight must be done within the overall weight limit of the vehicle and the axles. These limits vary depending on the distribution market and local regulations. Some markets do allow for an increase in axle load limits via a process.
Hitch Downforce.
This is less straightforward because it also moves some front axle weight to the rear axle because the towball is behind the rear axle. Towball downforce increases when towing downhill and during braking because the trailer pitches forward and leans on the towball. If you only tow occasionally I would do a front and rear spring upgrade based on weight gain and axle split then add airbags to manage towball downforce and rear suspension sag.
Opinion only. This is not a textbook.
