Remeber having the drive shaft dropped on an Amazon in my Mom's driveway, convinced I had a bad tranny. She wandered out and suggested that maybe it was the gear shift lever. I pooh-poohed that and continued working (and swearing). Last job before undoing the bolts was to pop back up and take off that lever. Jeez, the cap that retains it was way loose. That was the problem.

Since then I've found, on lots of vintage Volvos, that it's not an uncommon situation. Maybe not your problem, but it's the first thing to check. Very satisfying feeling when you find out how simple solving a big problem can be - unless you've already disconnected everything underneath, like I did that first time..