We wouldn't have to trade back to the late 20's to get 2 second rounders. In fact, St Louis has 2 second round picks, and with them sitting at pick 18 in the first, us trading back to 18 would get us both of their 2nd round picks. According to the pick value chart, it'd be a dead even trade.
They desperately need to find a #1 WR, and depending on Chris Long, might need a DE.
The only thing that throws a wrench in this is that Jeff Fisher is usually very frugal and conservative regarding the draft. I highly doubt there would be a player that he'd be willing to give up that much for.