Feels like bad U Joints but not

1976 Duster With 833 and 8 3/4 from a 69 Dart 340. Trans had new bearings 5 K miles ago and rear had new Moser axles and axle bearings 10k miles ago.
When I got home last week it felt like had a bad U joint when I went to put it into barn. Did not have time to look at it then. 2 days later I pit it up on jacks and ran slow and nothing and ran it up to 125 and still smooth.
Pinion has about 1/4 inch play at tire and no preload. I did not think this was bad for a 69 dif. I am thinking either carrier or pinion bearings when warm. Really never seen this with no other problems.
Also rear was a pint low on lube. Amsoil 75w90