I'm hauling a lot of grease these days and have a 12 gallon aux. tank in the trunk. When I put a few QB's in the back the back drags,,,bad. What shocks should I replace my old ones with and should I do the springs while i'm at it or not worry about it?

The standard for shocks is a set of Bilsteins. Normally everyone is satisfied with them.

As for the springs, you could consider a custom spring that is a bit stiffer and a bit longer than stock, if you don't mind the additional height when unloaded. It's always a compromise between unloaded height and loaded height.

A less costly way to go would be to install the thickest available rubber spacer above the rear springs. This would set the vehicle at a higher starting point when unloaded.

The custom spring is about the same cost as the OEM springs.

You can get them from these folks:

http://www.coilsprings.com/

Of course, I've seen the Bilstein on ebay, but is there another place to look for these and how much can I expect to pay?

I would recommend a set of Bilstein HDs. I have them on my 300SD and find them to be the best in terms of compromise between comfort and handling. The "comfort" Bilsteins are too floaty in my opinion. Same feeling goes for BOGE shocks. As for springs, just stick with stock MB springs maybe with thicker pads in them. The pads have little "tits" on them (1, 2 or 3) which indicate thickness. Go with the three tit pads.

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Regards,
Aaron