8.25" pinion bearing preload varies during turn

I have a "new" (from salvage yard) 8.25" sure-grip on jack stands, and I believe the pinion seal was leaking. Before changing it, I went to measure the torque required to turn the pinion. It varies from about 3.5 in-lb to 7 in-lb going around, and sometimes takes more like 10-11 in-lb to get moving. Is this normal? From what I see online, it should not vary at all. The variation occurs at the same angles each time the pinion goes around, so I'm pretty sure it's the pinion itself, not the axles or anything else. Possibly related: every once in a while (maybe every 20th rotation or so) the pinion catches and needs much more torque to get past the catch. I don't see anything obvious like a metal chip in the gears. This is my first time taking apart a differential, so I'm not sure how to proceed.