Anybody ever have an axle make a strange ticking noise? What is it? It sounds like paper or plastic wacking something as the driveshaft spins. When I put it up on jacks, I carefully looked under there with the motor running in 1st. Sound pretty much went away, but putting the weight on it, the sound came back.
I pulled the rear brake drums and for sure it's not there. It sounds like its at or near the input pinion on the diff (where the plastic is). The plastic "bezel" doesn't appear to be moving at all.
It sounds like a bearing. You may have some bearings going bad somewhere inside. Does the pinion move up & down at all when you try to wiggle it? The pinion nut also has a habit of backing off, but that happens more frequently on the 8.8" axle, not the 10.25" that you have.
BTW, your axle is definitely the sterling 10.25, and not a Dana. Whether it's a full float or a semi float depends on whether the hubs stick out through the center of the wheel or not.
I've had this truck for almost 2 yrs. Two days ago, I had a shop replace the differential fluid for the first time in my ownership. Could that be related...? Maybe they just used whatever was on the shelf.