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I contacted an online dealer for heavy duty replacement leaf springs a while back hoping to cure the weak rear springs on my van that I was towing with at the time. He was also a dealer for the Timbren helper springs, which he recommended to me over his more expensive replacement springs.
I didn't buy them, so I can't report results, but I thought his recommendation, in light of the fact that he sold a more expensive alternative, was interesting.
I put a set of Timbren's on the rear of my '82 E350 and it really helps when laoded down. Less sag and less 'wallowing' in general make the ride feel much more controlled and stable. I was attracted by the price, ease of installation, and never having to worry about an airbag blowing out. I imagine that overload springs would give one the same benefits as the Timbren's, while airbags give you more adjustability...
btw: i like my Timberlin boots, too. but that's for a different forum... :-)