Any idea what could cause this???? 86 k5 vibrating
 

My 86 Chevy k5 blazer has had a really bad vibration lately and i cant seem to figure out what is causing it at this point. i had run low on gear oil in the rear diff due to a very slow leak i was unaware of and that was the first thing i checked (vibration started some time after this). All of the gears seemed to be in good condition. I then found a bad U-joint and replaced that but it still vibrates. it only does when you accelerate over 30 and once you let off the gas it goes away. Someone suggested the transmission however i don't have any problems with it shifting or slipping so i don't think that could be the problem although i could be wrong. i have no other obvious drive train issues i can find so anyone who can help me out in diagnosing the cause of this would be helping me greatly. thanks.

Re: Any idea what could cause this???? 86 k5 vibrating
 

My dad had this exact problem on his 1983 K5 and turns out, the rear end was the culprit.

Re: Any idea what could cause this???? 86 k5 vibrating
 

How much 90w did you have to add when you realized it was low? The oil bath cools it, I'd suspect the rear end before the transmission.

Re: Any idea what could cause this???? 86 k5 vibrating
 

I had to add roughly a half a bottle. If it is the rear what else besides the gears inside it could be bad?

Re: Any idea what could cause this???? 86 k5 vibrating
 

You May have a cracked flex plate. It would cause exactly the vibration that you described.

Re: Any idea what could cause this???? 86 k5 vibrating
 

Which u-joint did you replace? If you did the one on the yoke it could be off 180*?

Re: Any idea what could cause this???? 86 k5 vibrating
 

The outer wheel bearings could be bad also, jack up the rear end and see if there is movement with the wheel vs the axle. These bearings going bad usually seem to be seen with axle seal leaks also. So you can also check the back side of the backing plates and the inside of the drums for gear oil. If everything seems good then refer to other suggestions like the u-joints/ flex plate. also take a look at your front drive shaft ujoints while your at it.

A bad tire can also cause vibration.

Re: Any idea what could cause this???? 86 k5 vibrating
 

Might want to get a angle finder and measure your driveline angles. Do this on level ground. Mine had a similar problem and it turned out my pinion angle was way too low. I fixed mine when I did the shackle flip when I lifted it. You want equal and opposing angles.

Re: Any idea what could cause this???? 86 k5 vibrating
 

could also be worn bearings, either pinion or axle. in addition to what russgt suggested see if there is movement on the pinion where it enters the axle.

Re: Any idea what could cause this???? 86 k5 vibrating
 

when I tore apart my front brakes and rotors I noticed the washer with all the holes in it was broken in pieces. I was tearing apart the front end to turn the rotors for vibratrion. I also noticed the pin that goes in that waser was sheared. after I replaced those two pieces and turned the rotors, the vibration was gone. It could also be real simple though, like the wheel weights came off one of the tires and now theres a vibration after 30 mph like you said. (unbalanced tire)