I went on a trip with my two sons and a fully loaded car out west. I knew the rear would be under extra load and the '95 124 E300D has a low profile stock as it is.
I inserted two hard 3/8" bolts and a nut on both sides of the lower rungs of the rear spring.
What I did was grind a nice groove across the bolt with a grinder on the head and screwed a nut on the other flush with the end of the bolt. If it's a bit tight to insert just grind the nut down a little until it fits easier between the spring rungs.
I believe I used 3/4 or 1" - 8 hardness bolt just slightly larger than the spring spacing with the rear end of the car raised up with a floor jack. I used a long metal pry bar or other 3/8" or 1/2" metal rod just enough to allow space for the bolt to pop in between the springs. It will sit there tight with a modest groove in either end of the bolt.
At the last I sprayed Ziebart over the bolt and spring to eliminate any possible rust when the paint is scratched off the spring.
This technique brought the rear end up to stock height and rode excellent on the freeway without bottoming on shock bumpers.
When I returned home I removed 1 bolt spacer in and left the other bolt on the car. One bolt spacer helps greatly with passengers or a trunk full.
I do not notice any ride difference.
PS, I did use air bags in my Olds Custom Cruiser Wagon springs and without any air it rode like a truck. I cut them out immediately.

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'96 C220 138,000mi, '95 E300D 239,000 mi., '87 300TD 214,000mi '88 6.2 Turbo Diesel Chevy Conversion Van 253,000 mi.