I posted this here a couple months back. I just reposted it again with the word transmission in the title so it is easy to search out.
Before you do anything else...Disconnect your battery! Clean the + and the - posts and the cables that attach to them. Check the chassis ground that goes from your batteies - post to the frame(chassis) and clean that too. Shiny metal to shiny metal is good. Rust and corrosion is not your friend.
I've followed alot of posts here that have transmissions in limp mode problems. Crawl under the vehicle with a haynes book in hand if need be, and check the solenoid connections to the trans. Clean the grounds for the solenoid! Check the wires for the solenoid! Too many trannys are rebuilt or replaced for big bucks because of a loose wire that was inadvertantly repaired during reinstall or a ground wire that was rusted.
get yourself an electrical diagram for your vehicle(
www.alldata.com) and learn how to read it and save yourself alot of money. These diagrams tell you where the grounds are for all the components. Clean them. If you can't decipher the diagrams, copy and paste them into word and email it to me and I'll be happy to help. I hate to see people spend big bucks for parts swapping when the real problem is a loose or bad connection. "Never under estimate the value of a good visual inspection".
Remember all the battery voltage travels out the -(neg) post of your battery, through the frame(chassis), into the load(motor, light, coil of a relay, or switch, through a fuse, and back to you battery +. (pos) post.