Cacti, per the manual, the Dana/Spicer S110 should be changed around the 5 year mark or 500k miles if using synthetic fluid which is what all the W20 and up came with. If you drive through water deeper than your axles, it should be changed immediately after.
My fluid came out crystal clear, and no shavings on the magnetic fill plug. I have 23k miles on my rig. Now that I have changed it, I don't believe I will ever change it again. My axle took 16 pints of Valvoline 75W90 GL5 synthetic.
I would just keep it topped off and your good to go!