My 07 has the 4R75E trans. I have always wondered what that rectangle block was inline of the trans fluid line. Is it an expansion valve, a heatsink, some type of proportioning valve, an inline filter???????

 

I think it's a thermostatic bypass valve, allows the transmission to warm up quicker.

 

Thanks GLC. It was exactly as you described. I have seen that thing so many times while under my truck and wondered what it was.
Something totally irrelevant to this subject; If someone says you MUST raise the engine (4.2 V6, 2 wheel drive) to remove the oil pan, YOU'RE WRONG! My oil pan was leaking very bad from the rear. I did pull the trans, thinking it was the RMS.

 

I think that was for the 1997-2003/Heritage model.

 

That is exactly correct.