Sounds like the high range clutch is not doing it's job. Of course, it is engaged by fluid pressure. All you can do is check fluid level, top it off, and try high-range again. If it comes back high-range, keep an eye on it. There could be other issues - some that only a pressure test will reveal.
I lost high-range in my first Packard on a 300-mile trip, bringing it home via I-79, just after crossing the WV-PA border. Stopped and found a leak at the front of the trans, but it was running out almost as fast as I could pour it in. I managed to baby the car in low one more exit, and called my dad to bring the tow truck. Turns out it was the front pump seal leaking. There were other issues, but I wasn't ready for a full rebuild. With only a new seal, I trusted the car for several summers of limited, local driving, but I haven't driven it much in recent years.
Ultramatic Dynamics is in VA. I'm not sure if he rebuilds 'em anymore, but the site still offers new, used, repro, and improved parts. There's another fella in the area, Ross Miller, who rebuilds.
However, here's a recommendation from one member from out west:
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=3559&forum=2&post_id=41117#forumpost41117 Sit back and see if anyone else chimes in with others.
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