Sounds like a plan, I've read into solonoid pack replacement but so far have not had to do it on my RE.

When you get it drained, take out the output speed sensor and clean the tip. If I remember right it's a small magnet and bits of metal will get to it. That may be why you are getting that code.

Don't take it out before you drain it........or you'll get a bath. Damhik.