Fifth gear transmission shudder

You are correct that this is the torque converter clutch that is slipping and is not fully locking the converter. Usually the only repair is to replace it if flushing the fluid does not work. However, there is one trick that you can try. Normally this only works if the converter is just shuddering and then locks up but by the time it is shudder all the time and not fully locking, it is too far gone. However, it is worth a try. I attached a picture of the Motorcraft version and the Mopar. They are called different things by different OEMs but it is the additive that goes in a rear end to break in the rear end clutches.

What this modifier does is soften the clutch material so it doesn't chatter. If you put a bottle of this in the transmission and then give it about 50 miles to work in, it should start to lessen that shudder. If it makes an improvement but does not get rid of it, you can add a second bottle but not any more than that because you will start to soften all your clutches too much which could lead to more than just a converter replacement. If you end up replacing the converter because this doesn't work, I would go ahead and flush the unit to get it out.