I have a 98 scab 4x4 with the 4.6 and the infamous transmission shudder at about 40 mph. Does anyone know if putting in a new ftvb will help?

 

Changing the filter and fluid might correct the problem. It really depends on how bad it is. If the converter clutch has failed nothing you do to it will fix the shudder until you replace the converter. That vehicle should have the best lockup clutch ford offers so it is one of the stronger converters. The early cars had a poor design lockup.
The FTVB will increase flow in lockup, but alone it might not correct your problem. Drop the pan and cut the filter open. If you find alot of red material in there the clutch has failed. Sometimes the material is black in the filter. Remeber always use MERCON V fluid in your 4r70w. DEX III will destroy the lockup clutch for sure. Ford uses a funny design clutch that needs the friction modified fluid to operate correctly. I am not sure what else in the transmission requires that fluid but I know the converter does.
Alan