when should differ. lube be changed, have 16 year old truck
Differential fluid is just heavy gear oil. I change mine every 100k mikes.
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When you begin to wonder if itâs due. 
Seriously, Iâve never seen a formal recommendation for diff fluid, which is typically either 75 or 90 (on old vehicles) weight oil, but it canât hurt to change it if youâre wondering. I used to change mine every now & again. just be sure you use the correct fluids, as itâs critical that the lube be able to withstand the compressive forces between the gear teeth and be able to properly adhere to the surfaces.
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Does it have a limited slip diff?
Good point! I had a Caprice with a limited slip differential and it needed the lube changed every 24,000 miles or so. And a special additive was necessary!
For regular units, just checking the level is OK, unless you have gone through water such as backing onto a boat ramp.
Limited slip differentials still use gear oil, so the interval should be no different then non-slip.
My Pathfinders and 4runner had limited slip differentials and I still changed them every 100k miles. Either get the regular gear oil and then add the additive, or buy the limited slip differential gear oil.
Actually LSDs often require more frequent changes because the mechanisms degrades the additive âpackâ.
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Definitely not from my experience. My user manuals also never stated a different change interval for LSD versions and non LSD versions.
I had a 1999 3/4 ton Suburban with a locker-type limited slip and a 2002 1500 Avalanche with the same type limited slip. Both required a fluid change at 500 miles and every 50,000 after that. The manual did not require a fluid change at all for open differentials in either truck.
My 2004 1500 Avalanche was delivered with synthetic gear diff lube - in the rear only, the front had mineral oil! - and did not require a change at 500 nor 50,000 miles.
GMâs Positraction units definitely needed more frequent changes. These units developed a lot of heat apparently, and a fluid change cost about $50. I knew a guy who did not change it and burned out the clutches in abut 60,000 miles.!
My limited slips were both ratchet types. The 3/4 ton Subbieâs was a traditional locker that kicked like a mule when it locked and the Avâs have âRev-Locâ or ratchet types with a speed disengage about 25 mph or so. Neither had clutch-type limited slips. Both are awful. I wouldnât want a clutch-type in a truck, either. Gear-type (Torsen, TruTrac) or open is my preference. The clutch types in my cars are OK to drive. I change diff fluid no later than 50,000 miles in those types. Earlier if they are used harder.
Do you have access to the Service manual for your truck? It should say so here. Older ownerâs manuals may have it listed in the back (people were more into DIY decades ago 
I change mine every 30,000 miles.
As The_Same_Mountainbik says, older cars tend to use heavier oil. I use 85w90 GL5 gear oil by LiquiMoly (1982 Cressida and 1979 Celica). The FSM actually recommends 90 weight, but try getting that!
I did mine about 150k, about 30 bucks a quart from the dealer for limited slip, generic parts store had the synthetic, then $12 per quart for the limited slip differential stuff, preferred oem!
Like @tom418 above, I replace the rear differential fluid in my Ford truck every 30K miles. I think thatâs what Fordâs recommended service interval for that model year is too, 30 K miles. If you ever drive through water high enough that get to mid-differential height, then water probably got in, so it should be changed immediately. Iâve had to change the diff fluid every day sometimes when off-roading and fording creeks all day.