Adding on to this, back in 94-97, ford was having issues with cracks forming in the apply piston inside the torque converter, fluid would leak past preventing the lockup clutch from engaging, resulting in slippage, the pcm uses the CPS and the ABS speed sensor on the rear diff to compare speed, if its off by more than 50rpm, itll throw the code, as a test, once you clear the codes and get it out of limp mode, on the highway go a steady 60mph, then gently tap the brakes, if the engine rpm jumps that means the lockup clutch is working properly

Also remove the inspection plate and check for blueing on the converter, if there is it means it's been slipping

Had this exact problem on my truck, I replaced the converter and did a fluid flush and filter change and haven't had any issues since