The trucks involved were built in a month-long time period in 2018, and Ram has identified the problematic driveshafts by part number. Apparently, the rear driveshaft may "have an incomplete weld fusion between the tube and tube yoke, which may lead to weld cracking or the driveshaft fracturing," according to the recall documents. This could cause the driveshaft to drop off the vehicle, and cause you to lose drive power (among other hazards involved with a driveshaft falling off a truck). At this point, FCA says it's not aware of any accidents or injuries related to the problem. However, it has seen eight occurrences of the driveshafts breaking, prompting this voluntary safety recall. Ram says that an extra noise or vibration could signal that the driveshaft is on the way out.
FCA is going to start notifying owners about the recall in mid-June. When you take your truck in, the dealer will replace your driveshaft with one verified to have complete weld fusion. It will, of course, be free of charge to the owners of the affected vehicles.