The rear coupler was noticeably cracked so that was what set me on the path to do those. Bought Meyle brand from FCP Euro for quite a bit less than the dealer. FCP said front and rear is the same. I'll get back to that.
First I did the shim mod. Evidently the sagging of the center support bearing isolator somehow brings the driveshaft out of alignment (by less than 1 degree - sigh). This can cause various NVH things but also a weird clicking that I was hearing in certain situations. Two stainless fender washers from Ace is all it took. Unfortunately they were out of 8mm washers so I had to buy 10mm washers. The thickness was just under 2.5mm so perfect. The bolts are E12 and torqued lightly to 15 lb-ft. Pretty easy work.
I did not road test just this change though I guess it would have been ideal to do so.
Next was the rear flex coupler. 8 nuts and bolts hold this in. To spin the driveshaft around I had to leave the ignition on for the car to stay in neutral which kind of sucks. Then you have to release the sleeves that sort of get rusted into the driveshaft (and diff pinion flange) by prying around. Then the rear section of the driveshaft has to slide forward because there is a guide piece that comes out of the diff. Not only does the driveshaft have to slide off of that, but it has to slide further so the flex coupler can actually come out. This required me to loosen the center support bolts (that I had just torqued) to provide additional clearance. I was just able to slide the old flex coupler out.
Good news: my new Ryobi power ratchet is exactly what I'd hoped it would be. T60 on that, back up nut with 18mm wrench. Each bolt typically took a few tries but it broke all of them loose and then effortlessly ran the nuts off obviously.
Bad news: the old flex coupler is different from the new flex coupler. The old one has extra beefy sleeves. It also uses a weird assortment of hardware. It uses two different length bolts and two of the bolts have this weird cap on them. This contrasts significantly with the included hardware of the new flex coupler which contains 8 identical bolts, washers and nuts. Initially I thought maybe I can somehow swap the sleeves over but that thought was quickly dashed after an attempt in my vice. So I thought, maybe the new one just bolts right in. Well, it does. But, after I finished the rear and torqued to 50 lb-ft, I looked at the center support. The center support has fairly long slots to provide for centering adjustment. The original position was probably 75% toward the front. After torquing the new rear coupler, the adjustment looked like it was all the way to the rear! This is because the sleeves on the original coupler actually added some thickness and pushed the driveshaft toward the front of the car a bit. This used up all the adjustment. I don't like this, but my hope was maybe someone actually engineered this to work. I moved onto the front flex coupler.
The front flex coupler was a bit tighter to access to the exhaust and a heat shield. I quickly realized that I could just bend the heat shield out of the way and then my sweet ratchet fit right in. Hold back the nuts with an open end wrench from the side. This beats having to drop the exhaust and a bunch of heat shields. Somewhat tedious (with 8 bolts). I noticed the front coupler now matches the aftermarket couplers I bought. So....the front and rear is not the same (ahem, FCP Euro). It was a tighter fit somehow to slide out the coupler (even though it was a bit lower profile). But I got it.
Slid new one in, insert bolts, torque, etc.
My plan for the road test was to see if there were any weird sounds just moving the car around in the driveway that would suggest undue stress on the center support (or perhaps other alignment issue due to the rear flex coupler having different sleeves). That test passed fine. So I took it out on the road. Still seemed good. In fact, I think NVH was actually reduced and that "tire noise" that started at 35mph seemed less, and now just tire noise rather than tire noise + something else.
Oh I did check the rear wheel bearings when the car was in the air and they are both tight.
Anyway, drove the car around and got it up to highway speeds, and touched 100 once since I had noticed some vibrations at that speed before. Good news: I think all that is fixed now. I will have to drive more but yeah I am happy. I am still considering sending FCP a note and seeing what they say. Should I try to obtain an OEM rear coupler and install that to make everything right? I can't decide.
Looks like this is the P/N for the rear coupler.
https://www.mercedesbenzpartsshop.com/o ... dhcw%3D%3D
And here is the front one:
https://www.mercedesbenzpartsshop.com/o ... dhcw%3D%3D
I think the best course of action is to first see if FCP will make it right somehow (like let me return and use that as a partial credit to the correct part) or I guess just return for a refund and I'll have to drop $250 on the OEM one. This is not something I want to experiment with. Unless they come back pretty confidently and state, yes, we know about the difference but it is acceptable to replace as I've done, been tested, etc.