Re: Towing with front wheel drive vehicle
I had a Lincoln Town Car that I towed my boats with. Fairly similar to the Merc you are using. Put in a limited slip rear differential, and problem solved. The best LSD is an all gear unit made by Eaton called Truetrac (sometimes called a Detroit Truetrac). They are silent, smooth, no chatter or grabby stuff. And no clutches to ever wear out. Have one in my Firebird, and its the best street LSD I have ever owned.
I now have about 45,000 miles on my Eaton Truetrac in the Firebird. Running 3.73 gears, it gets a good amount of torque fed into it. Nonetheless, it performs BETTER today than it did when brand new. At about $400 from Summit Racing, (install at your own cost) it makes the tail wagging firebird behave much better, especially in rain, snow, and even WOT on dry surfaces. When the unit locks (and it will lock up after a certain amount of torque is fed into it) its just as fun as the classic Detroit Locker.
Now, the Town Car did not have the Eaton Truetrac, it got the standard Ford factory clutch posi, and after about 60K it was time for new clutches. But for pulling a boat up a wet slippery ramp, the Town Car (which has the same engine, trans and rear axle as your Merc) never spun a tire once a LSD was installed.
So if you are going to put in a LSD, go with the Eaton Truetrac. And only use pure synthetic gear lube. (its a small price, less than $20 to drain and fill the differential on your own).
Here is a cool video that shows the inner workings of the differential.