The converter will not drop down if you remove the inspection cover and remove the torque coverter bolts. They should be match marked but who knows. Just mark it so you can put it back the same way; just in case.
Just curious. Which engine do you have? There are a number of engine options and they all use different harmonic balancers/flexplates/converters.
The 5.0 even has 2 different balancers (28 and 50 oz.) available depending on year and application.
Normally, that rumble strip vibration is a torque converter shudder and fresh fluid (Mercon V) will cure it or in some cases a new converter is needed. Since they say they put in a new converter what is the story behind the transmission?
Is this trans an early non-electronic AOD? What year? New, rebuilt, or used transmission?
I'm assuming this swap was done to improve fuel economy.
The details behind the balancers/flexplates/converters are pretty fuzzy to me so you might consider fishing around on the net this weekend for more detailed info.
I ran into an early Ford AOD trans swap vibration problem many years ago but I just don't remember the details behind it. Seems like a late 70s OD transmission was not working well with a late 80s V-8 and had a subtle, but chronic, vibration in it.