lately i've been noticing a shudder from the rear of my blazer (see signature) on takeoff. it wasn't too bad at first, but quickly got worse and now it makes the whole truck vibrate constantly. there's also a very low pitch rumble or howl at highway speeds (sounds like excessive road or tire noise, not terribly loud but loud enough). coasting in neutral with the engine off produces the same result, so the engine and trans are out. i kind of figured it was the rear differential, so today i crawled underneath and sure enough it's leaking from the driveshaft seal. i've read that there's a plug on the case, but haven't had a chance to look for it yet to check the fluid. what kind of lube does this thing need? i really wish i would've noticed this sooner, this truck is very quickly getting very expensive.


edit:
found this thread, is it correct for my '97?
https://blazerforum.com/m_21670/tm.htm

 

Yup, that post will work for you.
You should see if there is any up and down movement in the pinion flange. Just try puhing the rear of the driveshaft up and down to check for play.

 

well, the rear diff was about half a quart low. adding fluid didn't make much of a difference, if any. i didn't get a chance to check for pinion flange play (dad looked at the truck while i was at work).

anything else to check out when i get a chance?

 

no play.

swapped the rear wheels with a known good set. no change.

none of the wheel hubs are abnormally hot after driving (in fact, the rear ones are cold to the touch).

 

If you jack up the truck and set it on axle stands, use a screwdriver as a stethescope (put the blade on the diff housing and the handle to your ear), with the truck running in gear, see if you can locate the noise.

I would suggest removing the diff cover and check for metal filings in the fluid and the housing. I'm thinking worn differential pinion and /or case side bearing wear.

 

we do have a mechanic's stethoscope, i'll see what i can do. it makes the most noise when accelerating, and currently you have to be going at least 50mph to hear much of anything. the noise is there at lower speeds, it just isn't very audible. we drove around last night and i sat in back, couldn't pin it down to one side or the other.

 

It really is starting to sound like you may have lost the bearings in the rear due to the leak... My Bravada had a high speed cyclical vibration when I first bought it. You'd have to be doing 65-70mph (interstates) to feel it. One good thing was the first trip I took with it was going out to my dad's. They ended up having to replace all of the bearings in the front differential along with the front ring & pinion.

Being half a quart low shouldn't have made that big of a difference, but it may have starved the pinion bearing... Have you checked for play in the pinion? How about in the axles? With the rear end up on jack stands, is there up and down play at the wheel?

I too would recommend opening up the rear differential and giving it a good once over. Take a bar and see if you can move the differential carrier. Do the same thing on the pinion if you can get something up to it to try to move it. If either of them move, you've got bearings that aren't right.

 

thanks, but that's really not something i wanted to hear. the vibration does seem a bit cyclical, but it's hard to tell (tried putting it in 4wd, too - no difference). we'll check the things you mentioned. i appreciate all the help, by the way.

i bought the truck last february with just under 96k miles on it (now has 109k). according to the shop that tried to align it, it needs new inner and outer tie rod ends, upper and lower ball joints, a pitman arm, and an idler arm. there's quite a bit of play in the steering wheel, and these are notorious wear items on these trucks, so i'm inclined to believe them. i just haven't set aside the money to buy the parts and a 24-pack of beer for dad. now it looks like i'm in for an even bigger parts bill. good thing the labor's cheap.

 

fixed. bad universal joint.