You might want to look into switching to a good quality name brand Mercon V fluid, I'm not sure if its compatable with your transmission or not but I have a '97 Thunderbird with the 4R70W automatic (overdrive) it started shuttering recently and the folks over at the TCCoA website suggested I change the fluid and filter which included draining the converter which I did myself, spent about $60.00 total and my shuttering is gone completely, the Mercon V fluid is part synthetic, and right on the bottle (Valvoline brand) is says "Recomended for all '96 and newer Fords"
So before you let anyone talk you into an expensive rebuild try the Mercon V fluid.