The driveshaft spun in an oblong motion

I replaced the transmission around 2-3 weeks ago. When installing the driveshaft, I noticed that the driveshaft spun in an oblong motion instead of straight. Due to improper spinning of the driveshaft, the rubber coupling/flex plate got chewed up and the metal support pieces in each bolt hole separated from coupling causing the driveshaft to separate from the transmission. So I went yesterday and got a new driveshaft and replaced that. While turning the driveshaft by hand and watching how rotate I noticed that it still spins in an oblong motion. What am I suppose to do to correct this problem so I don't continue wear/tearing the rubber coupling/flex plate apart? Also can u send me a link or video on how to properly check and fill transmission fluid? (The image is from the tow truck driver right after it broke)