The reason we don't usually do a filter/fluid service when the fluid is in real bad shape is because usually you can't drain the fluid in the torque converter and when the old burnt fluid from the converter mixes with the new you just put in the trans it can get ugly. Most of the rear wheel drive Fords come with a drain plug in the converters so you can remove all the fluid when servicing the filter, especially if you remove cooler lines and blow the cooler out. At the dealership I have taken quite a few Grand Marquis, mustangs, etc. with fluid so bad they shuddered going into overdrive, removed the pan and filter-drained the converter-and flushed the cooler, and they were fine afterward. And for the cars that don't have a drainable torque converter we use a power flush machine.
BUILT FORD TOUGH