How do I get the back off of the washer?

You don't remove the back; you remove the front. The front, left, and right "walls" are all one piece, and that piece will come off easily if you do it correctly, giving you access to everything inside of the washer.

As to the cause of the problem, my guess is that your coupler is broken. The coupler is a plastic/rubber piece which connects the drive shaft to the transmission shaft. It is designed to be the weak link, so that if the washer is way overloaded, the coupler will break, thereby preventing a more expensive part from breaking. A coupler will cost you less than $20 at your local appliance parts store. I had a machine in which the coupler often broke, so I kept an extra one on hand, in case it decided to break at night, after the stores were closed. And it took me about 15 minutes to change it out, once I did it a couple of times.

There are a lot of YouTube videos which will very clearly explain how to remove the front and how to replace the coupler; I suggest you watch a few of them, and take some notes. I will summarize the steps below.

Before you start, unplug the washer from the electrical outlet! Turn off the water faucets to the washing machine. And take notes and pictures as you are doing this, so that you will know how to put it all back together.

1. Remove the top part, the part with all of the knobs and controls. You should find a screw at the bottom left corner, and another one at the bottom right corner. Using a Phillips screw driver, remove these two screws. The top should now tilt backwards, out of the way. There will be a wiring harness inside of the top piece that you will need to unplug as you are doing this, so as to allow the top part to tilt all the way back without damaging the wires..

2. There are two large clips holding the front part of the washer. They should now be visible, now that you have moved the top piece out of the way. You should be able to insert a big flathead screwdriver into each of the two holes, with the blade pointing to the rear of the washer and a little bit down. Once you have inserted the screwdriver all the way, push down on the handle until the clip pops loose. You can now remove the clip. Be careful not to lose the clips!

3. Now that the front part is free from the rest of the washer, hold it by the back of the washer, and tilt it up. The front end will stay put, but the back end will come up. Once you have it up a bit, you can lift it up and away from the washing machine.

4. With a flashlight, look under the washer. I believe the pump is right there. You'll need to disconnect it and move it out of the way. I don't recall if you need to unhook the hose from it.

5. You should see the motor right in the front. Carefully unplug the wiring harness that is plugged into the motor. The motor has some clips holding it on. You'll need to remove the clips with a flat head screw driver. But be very careful, and go slow. You don't want the clips flying off, and you don't want the motor falling off when you remove the clips.

5. Once the motor is off, you should be able to see the coupler on the shaft. If it is broken, it will need to be replaced. You can use something like this to get it off:

https://www.harborfreight.com/9-3-4-quar...

Basically, something small that can reach around behind the pieces of the coupler to push it out and off the shaft.

(Don't get a plastic/metal coupler. They are very hard to remove from the shaft if they break.)

Take the coupler pieces, along with the washer's complete model number, to an appliance parts store, so that they will know which coupler to give you. (There probably aren't that many to choose from.)

When you put it back together, you will put one piece on the motor's shaft and the other piece on the transmission's shaft. You then have to get the two pieces (with the rubber part in the middle) to line up, so you can get it all back together. Putting it all back together is the reverse of the above.

Be sure to watch those videos before starting!