Sorry to hear that on your daughter’s truck. That is simply unacceptable. It just can't be that hard to put these things together reliably after decades of the same essential components.
Rear main seal is the verdict, looks like a 2-3 week turnaround. Dropping tranny and getting into the rear early next week. It’s recoverable but definitely sucks on a new rig, thankful it’s not a cracked block like they had for a while or engine replacement type of problem.
Been following a few Colorado forums and looking like a window opened this summer with bad rear main parts and/or assembly as they’ve been showing up randomly since late June.