I fixed my broken arm rest today, in large part to the YouTube video (link below).
There's a pin that keeps the arm joint in place, it fell out. One of the pictures shows the hole with the missing pin. Thanks to the YouTube video I was able to figure out a fix. I bought a 3/8 inch (turns out to be the right size, or at least close enough) round steel rod from Home Depot, cut it to the right length (not much more than an inch), and glued the new pin into the hole with Dap Rapid Fuse. - Problem fixed.