That's pretty good considering the loads you haul!
At the time all I replaced was both pinion bearings.. everything else looked good.. so re used them.... I put a mag hytech cover on was running 85/140 conventional oil... Started talking 200 miles into a 1500 mile trip at 36,000lbs.
Since the oil change in Walmart parking lot, only had 80/90 and put a bottle of Lucas hub oil in.. very little to no shavings on dipstick magnets