Yeah - I thought it was strange, too, when the same thing happened to my Dad's '97 GL 3.0 Vulcan a few years ago. His mechanic did not properly maintain the cooling system, it sludged up inside, a freeze plug rusted out near the power steering pump ($90 to fix), then later came the gushing leak similar to what you've described and he was told another plug had rusted out behind the tranny. Cost him $700 to fix it and he dumped the car right afterwards, disgusted with Fords. Another example of how bad servicing can ruin a manufacturer's good name. Yes - these cooling systems can rust up on you, but if you stay on top of it with flushes and refills, you can avoid a $700 bill to replace a $5 part.
I'd shop around and see if you can get a lower number to do this job. And I'd make sure the right plug gets installed. A lot of times the rubber plugs are installed (because they're easier to install, I think). Don't know if they last as long as the metal ones from the factory, or if you should even install a rubber plug in this location.
If this happened to me, I'd be tempted to try fixing it myself - seems like you'd just have to lower the tranny enough to expose the plug, but maybe just doing that is difficult for a home mechanic?
'97 Taurus GL Wagon 3.0L OHV
226.3 miles (03/12/16)