Pull the driveshaft and take it to a shop. You can't diagnose this on the web.
If the driveshaft is fine, tell
not to drive 5-15 mph on her 4K trip to the east coast and she'll be fine and will actually get there as well
We also do this trip every year (not this year, thankfully). Freaking long drive in a minivan with dual DVDs, that's death in an 80.