I have a 2004 ranger fx4 4x4 4.0l, when I start driving the tranny seems to work good until it gets warm, about 25km to 50km, then the O/D light starts flashing. So after some research I pulled off the valve body l, the gaskets were the original bonded gaskets to the separator plate. they seemed to be in good condition and I only ordered the individual gaskets and I wasn't sure how to get the gaskets separated from the plate. I Installed a Transgo shift correction kit for the 5r55e , and I tested the solenoids with multimeter and the readings were all within range. so I changed the filter , gasket and fluid . after a test drive it shifted better but the same ended up happening after it warms.

 

Any updates to your issue? Did you find out how to fix it? I'm dealing with a similar issue