my guess about the plastic in the trans filter is going to be; it got sucked into the trans from around the U-joint, you should check to make sure the operators were not using the 'vacuum cleaner' functions, by driving over plastic shrink wrap rather than picking it up. there is no 'plastic' that is native to the inside of this transmission. you are going to have to get this truck up high enough to get underneath and check the U-joint. make sure to check all the bolts are torqued properly on the U-joint, this U-joint turns at a pretty fast rate, and does sometimes let go if even 1 bolt is not properly torqued, I always add red locktite to the bolt threads if they are ever the least bit loose. The U-joint often takes the transmission and/or differential housings with them when they fly apart, meaning the truck needs a trans and diff replacement with no good core for rebuild (high dollar). but it is about the only place plastic can get up inside this trans.