astro rear end noise

My96 Astro 2wd is starting to have a slight humm/vibration from the rear. 115,000 miles and I checked the differential oil and it was not too low but when I topped it up some of the old oil got pushed out and it was filthy. This is irritating cause I am good about all the other maintenance and was told that this didn't need changing- "good for the life", or something.
With the rear end jacked up, there is what seems like some more than usual in and out play when I push/pull on the wheels. So maybe the gears are getting worn.
Thinking of flushing the old crappy oil by draining as much as possible and filling with new, driving a bit, and then doing a full change again, this time with a new gasket, etc.
Any other possibilities or recommendations?

The theory of "good for life" will shorten its life, guaranteed. Axle fluid should be changed regularly like any other fluid. The owners manual will tell what the recomended mileage interval is. 36,000 miles is probably in the "ballpark". Remove the differential cover to drain the old fluid. Flush with clean axle fluid, install a new gasket on the cover, and refill with fluid. If the differential is noisy, changing the fluid may keep the wear in check for a while, but the damage has already been done. The howl noise could be caused by worn bearings, gears, or a combination of both. A small amount of axle end play, usually about 1/8", is normal.

The astro van are bad about the bearings going bad. whine is the gears hum & vibration is a bearing. GM will use any brand bearing they can find.:nono:

thanks for the advice; I changed the oil and it wasn't as dire as I thought- the crappy stuff that first came out was a sort of frothy foam on top, and under was regular gear oil, filthy but still there.
I'll check out the brakes and bearings next.

Digging this one back up. My 93 whines pretty bad. Last year I replaced the outer bearings and seals. I didn't think to check the axles for pitting or even see if I could move them from side to side. My question is, "Is it common for those outer bearings to go bad and wear into the axles causing this humming?" I will put it on some stands and use a broom handle or stethoscope. If it's the outers, I can replace the axles and bearings. If it's the differential gear, pinion and all of that I am pretty much screwed. I think a replacement or rebuild wold be in orders. At 200K, I don't think the van is worth a 2 to 3K investment to repair as one shop told me.

Corndog...

i dropped the pan on my 88 and found a piece of a gear on the magnet inside the cover.. it was humming for 6 months....i went to the junk yard u pick and u pull... got a rear from a 94 converson for $90.00 drum to drum......did the work myself over the weekend....had to replace the rubber line from the body brake lines to the rear $10.00 and i made new lines on the rear to $20.00...just cut the wires off from the wheel speed sensors...used my brake shoes and parts and got a box of spares parts and drums......