Stock 06 w/130k on the clock with two rear axle fluid changes, one at ~60k and second at ~120k.
Have noticed since the 110k mark the axle tends to act more like an open diff and less like the anti-spin it originally was years ago. It's been a gradual change and will occasionally still operate in anti-spin mode.

Are these diffs rebuildable?

 

Yes, they rebuildable. It's not cheap, but a common repair. Remember that these diffs are not the standard LSD with clutch packs. It's a modified Gleason worm gear, no clutch packs. Very similar to a Detroit Gear TruTrak. The torque bias is adjustable during install. You might be able to re-adjust torque bias - IDK.