Differential fluid change schedule?

[KMR]

So today I brought my 08 Explorer in for an oil change. I get a call to pick it up and the owner tells me that diff fluid looks dark and has metal shavings! Really....I am pretty savy when it comes to cars....for yrs I always changed my own oil, but with work and kids just don't have the time and easier just to take it in. Anyway how is it possible to see metal in the fluid without draining it.....Am I nuts and missing something? I am always skeptical of shady auto shops (in my younger days I pumped gas and saw the tricks first hand, the mechanic used to have me write the repair receipts, lol). So I called my bro in law and asked him his opinion. (He owns 4x4 rock crawl axle business). He reassured me that at 27,000 miles i didn't need it done. What I am asking here is when do I need to do it? Also is there anyway to see metal shavings without draining the fluid.....lol??? Thanks