I would be cautious about using lifted (longer) heavy duty springs, even from Eibach. The failure rate of front driveshafts is anecdotally higher in lifted vehicles than standard height vehicles.
Given you're only seeking an increased spring rate I would avoid a lift kit - at least until the driveshaft CV failure mode is better understood.
If you fit a set of heavier springs from the factory options (chart below) your warranty should be unaffected. See what colour code springs you already have fitted. You may be able to upgrade to heavier rears. Black triangles is the highest rear spring rate available from the factory.
If your load is constant then upgrade the springs. If you occasionally run at higher weights then rear load assist airbags would be a good choice to restore the lost ride height due to spring sag without living with a stiffer ride at lower weights.
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