My experience with the stock setup and satisfaction with the Koni "special" red rear-shocks-only solution mirror those of DidHeFocus. Unacceptable ride characteristics were eliminated.
Most of the topics at this site relate to modification for pleasure (and as someone who street-rodded a rabbit 25 years ago, I understand completely). This one is about rectifying what I think is a plain engineering defect, and probably making the car safer for every-day, get in here, get out there type driving...the kind of driving for which wagons are presumably bought. Some of the posts earlier in the thread are obviously from owners of other models who have no idea just how bad the wagon's heavy-load syndrome is in the rear end even at curb weight...the engineers who signed off on the stock design should have been fired and Ford should have at least boosted the specs for the production dampers drastically...but apparently the company has no shame, so the only solutions come from third parties. It's so bad you don't actually have to drive the car - just lean on one in a parking lot somewhere and watch the rebounds F&R and you'll see...it's as though the shocks are new in front, 20 years old in back. Unloaded.
Anyway, I got the wagon at 4 years, 33k miles and my first thought was that I had early rear shock mortality. I scored a pair of 5k takeoffs (from a seller with superb feedback) via ebay for a song, but when I went to do the swap I discovered they were no stiffer than the ones on the car...my conclusion, particularly supported by this thread, is that I was getting the ride that ford intended! I spent a few months wallowing down the road, waiting to read something more definitive about the KYB GR-2s in this application (because they are cheaper than Konis and who wants to pay for more than they need), but finally I couldn't wait any longer - and after the ebay wrong-turn I desperately wanted to avoid the possibility of undershooting the mark with GR-2s, so I took the plunge for the Konis. I too have left them in the mildest setting, which feels about right in the sense of being almost harsh while alone in the car but without any of the wallowing with a full car that the stock shocks delivered even in a low-load, driver-only situation. If this car was ALWAYS to be driven more fully loaded I might increase the setting, but as far as I can tell so far, I won't need to.
Installation notes: it helps to have an assortment of extensions, and a six point 15mm socket and a cheater stick are necessary because the bolts are torqued down pretty good (the nuts are captive/fixed on both upper and lower attachments). I spent about half the total installation time figuring out the wrench/extension layout/routing required to break the upper bolts loose. Asymmetry in the layout of interfering objects means different wrench routing for for L and R - at least it did for me. I did this with the car on the ground, with one wheel up at a time, and others chocked, but I'm still pretty skinny so this approach may not be feasible for everyone.
Product note: before purchase, I thought that the Konis would allow me to field-adjust from underneath the car by releasing the lower fitting and fully compressing the shock in order to engage the adjustment mechanism. But the adjustment cartoon that came in the box also shows that nested high under the cap is a rubber bump-stop which one must push down with a probe via a hole in the top of the cap and then remove from the shaft (this rubber part is split, not a true ring) before the shock can be compressed fully enough to adjust the setting....and I neither had the guts to toss these before installation nor can tell whether they can be plucked out from underneath when the wheel in question is unloaded. Probably some well-positioned dental floss would have made this feasible without creating interference between the cap and outer surface of the shock body when the wheel is loaded - on the wagon, access to the hole on the top of the cap seems hopeless once the shock is installed. As a result, field adjustment of the setting will be a lot more work than I'd hoped, and vastly more than for Koni struts with the little knobs on top....it's not something you'll do three or four times a year to suit particular payloads or road conditions for a particular locale. In my case, the out-of-the-box, mildest setting seems right, but I admit that I haven't driven around at GVWR, as seems inevitable at some point given the size of the cargo area.
Bottom line: my experience sheds no light on the KYB GR2 vs Koni 8040-1268 decision, but if you have a wagon you should stop waffling and get a pair of one or the other product for the rear end. You *will* wonder why you waited so long. Thanks to DidHeFocus for reporting on his experiences.