I'll go ahead and reply to my own question with what I found, just in case anyone is Googling for information on a similar situation with this model. I was really trying to limit the amount of down time and number of times I had to disassemble the dryer for this repair, so I ordered the 4 bearing slides, the rear bearing kit, a new belt, and the idler pulley. Once the parts arrived, I took the front off the dryer off, and realized that I was going to need the upper drum support bracket also (part WE14M124). The drum had completely worn through the original bearing slides, and on the left side it had completely worn through the support bracket also, so there was nothing left to install the new bearing slides to on that side. So, I went ahead and replaced the rear bearing kit, and installed the new belt. I ended up not installing the new idler pulley because with the drum out, I was able to inspect the existing one and it seemed just fine--no signs of wear or difficulty spinning. I ordered the new upper bracket, but we needed to do a couple loads of laundry over the weekend, so I installed 2 of the bearing slides on the side of the existing bracket that was still in tact enough, and put the dryer back together. The dryer worked with just the two bearing slides, and the squeak/squeal noise was gone, but it still made a rubbing noise. A couple days later, the upper drum support bearing arrived. I took the front off and replaced the drum support bearing, and installed the four bearing slides. Reassembled the dryer, and now it turns very smoothly and quietly.

Bottom line, I think replacing the rear bearing kit was probably necessary. The 4 bearing slides were definitely necessary since they were completely gone. The belt maybe wasn't needed, but since I was taking the drum out anyway, I don't think it was a bad idea to replace it. The pulley wasn't needed. I could have saved myself some trouble and maybe a few bucks in shipping if I had inspected the situation before ordering parts and realized that I needed the upper bracket also. But, it worked out pretty well in the end, and the dryer is no longer noisy.