My truck started slipping going into first, prompting me to change the tranny filter and whatever fluid was in the pan. It had also been slipping when the torque converter locked up. Both problems disappeared, however, what I did also changed the chemical makeup of the fluid.
A few months later, the converter shudder got really bad. I added a tube of Dr Tranny instant shudder fix and the shudder was gone. Some of it returned, so a month later I added a second tube. I get a slight bump when the converter unlocks, but eveything else is silky smooth.
I'd try additive first because you don't wanna blow $100 on fluid if the tranny is going out anyway. It's not a permanent fix, though. Ford says that the shudder is caused by old fluid, so if the tranny turns out to be OK, you'll may want to change all of the fluid and the filter. I used Valvoline fluid, next time, it'll be Motorcraft.
The only exception is if the tranny is whining, then I think you should skip the additive and change the fluid and filter. Mine was whining before I did that, after that the whine and slipping in first gear were both totally gone.
I have a 4R70W, by the way, and it has over 200K on it.
