Well if you raised the rear air bags that does increase the pressure in the system on the rear, that's the only reason I can see it being "excessively stiff".

When I bought my X5 (I have 2-axle air suspension) the ride height was calibrated 1.5" HIGHER than stock height. Needless to say, I was used to it, but I also was ecstatic at the improved ride quality when I calibrated it to be stock height.

Lower that axle to the proper height my friend in the rear and it should be better, and measure from lower wheel edge to fender, I know for 20s with sports package is 709mm in rear, not sure on your wheel size, but here is a chart:


_______________ 17"____18"____19"____20"___these are wheel sizes
front axle_______685____697____710____722
rear axle________687____699____712____724
sport fr_________670____682____695____707
sport rr_________672____684____697____709

__________________
2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards