Weird, scarey sound! Please help!!! on 1990 Dodge Ram Van B250

I'm hearing a vibational sound when I take my foot off of the accelerator, while slowing down. If I start braking, the noise disappears, & no noise during acceleration, only occurs during de-acceleration. (Recently had CA. smog check, passed, but started immediately afterwards. Coincidence?) The sound is similar to the sound you would hear while de-accerating with snowtires on in the summer, only it's getting louder and with a stronger vibation each time I drive it. (It's only been a few days.) Help, PLEASE!!! THANKS!

Asked by for the 1990 Dodge Ram Van B250
It may be one of the tires. Inspect all of them looking for cord separation. Very common. Also the differential oil. Good luck.
Thanks very much for your assistance! By "cord seperation", do you mean seperation between the treads of the tire? Also, how do I go about checking the "differential oil"?! Thanks again!!!
The tire will have a bulge where the cords have pulled away from the carcass, it may not have split open to expose the cords but the tire will still be out of round. On the differential you might see a rubber grommet of the back of the diff cover. Gently pry that out with a flat screwdriver and may sure there is oil filled up to the top. If you don't see a rubber grommet then check on the side of the differential itself. The plug should be half way up the side that will accept a 3/8 extension. Remove and make sure the fluid is up to the top. Wipe the gear oil on a white paper towel to see what color it is. It started out dark amber, if it's black get it changed.
Hi Hal!
Sorry to bother you again, but while checking the tires, I noticed a dark puddle leaking from just before where it crosses with the rear axel. Looked like old dirty oil but smelled something like rotten raw onions! Pungent! I don't know where the "differential" is, but perhaps I'm getting close?! Is it drivable or do I need to get it towed to a repair station? Any idea of what this type of repair would cost? Thanks for your patience with me asking so many questions, but you've been a real Godsend!!! I truly appreciate your help!!!
P.S. Do you provide shuttle service back to Richmond? (Hint hint!!)
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