380sl rear end clunk?
 

When I get up to speed then take my foot off the gas there is a clunk that comes from the rear end of the car. Also when I slow for a light when I take my foot off the brake pedal it will clunk then. Am I right to think this a lash problem in the rear end, or could it be a u-joint. Anybody have any thoughts?

No U-joint back there, there is only one in the center.

Check the flex disk on both ends of the driveshaft, chances are one is bad (a thump on downshift is a common symptom).

Do not delay, if it's bad enough to thump, it will fall apart sooner or later, and the driveshaft can do some serious damage if it comes out!

While down there, check how far you can turn the driveshaft before the wheels turn (car jacked up). If more than half a turn, with a clunk from the diff, the dif spider head is worn out and you need to replace the diff. Flex disks are more likely.

You should also inspect the differential mounts, rear subframe mounts, rear control arms, and rear control arm bushings. If the bushings are bad, you will be able to move the rear wheels back and forth with them off the ground. you will also have noticable torque steer.

Peter

Ditto to what psfred says; you can often repair the diff. in a smaller-engined car, however, by replacing the plastic spider idler gear concave spacer bushings with the upgraded metal concave bushings they now sell. You will probably want the spider idler gears as well, and you will also need a new spider idler gear shaft/pin. Oh, yeah, it's not really a DIY job, unless you have equipment to measure backlash and friction - you can do it w/o all the fancy equipment and get it close enough, however; just keep in mind that you may have to re-space the primary spider gears if the new idler bushings are too tight (measure your old spider shims and order the next 2 sizes down to be safe). Pay attention to how it comes apart, because it will need to go together the same way -- mess the assembly order up, and you'll destroy your ring/pinion gear assembly (meaning bye, bye differential).

You could also have an axle acting up (if the outer fastening bolt comes loose, or if the sealed ball joint is going bad); also the flywheel plate can do the same thing in case everything else checks out (has more of a high-pitched, tinny sound)