Differential Unit problems of the 2015 Subaru Crosstrek

2015 Crosstrek. Was driving on i476 in PA on a 35 degree sunny day on 2/6/2021 for about 3-5 miles at steady speed of 65 mph when suddenly the car bumped (although nothing was in the road/no pothole) with a loud noise (nothing flew off car) and abruptly slowed to a crawl even with foot still on the gas. Luckily we were in the right hand lane with an open shoulder to swerve into otherwise the person behind us likely would have slammed into us (seeing as no brake lights would have gone on since foot was on the gas). Tires, underbelly and under hood looked fine. 76k miles. Never had any issues with the car whatsoever other than rear wheel bearings replaced due to age a few months prior. Otherwise just maintenance per the maintenance and warranty manual. No issues or indications of any problem leading up to it and just had the serpentine belt replaced just due to age (precautionary) the day prior. Had mechanic generally inspect car the day before when belt was being changed because planned to drive to nc the end of that week. Called aaa who (after 2 hr wait) towed to our mechanic. Our mechanic spoke to concordville Subaru (melissa b. ) and we had it towed there. Were told by concordville, �transmission needs to be rebuilt. The gears jammed in the transmission and some broke and there was some collateral damage. The warranty is extended so it should be covered and I'll work with the Subaru inspector to have the collateral damage covered. � the next day when they were told by us that we had the differential fluid changed (per Subaru maintenance schedule) they said it's probably a differential issue so won't be covered and if we want to see if there is a transmission issue it is $1000 all in for diagnostics and $8k for a transmission rebuild if it's not covered �and I doubt the inspector will approve more than 25% being covered. � (melissa b. ) (via email).