So i got a used third member for my 85 and it vibrates before it (disengages?) at speed above 50.
Tire pressure is good, driveshaft bolts are tight.

At speeds above 70 the truck starts to shake me outta my seat. Progressively louder thunder vibration at 80. Letting off the gas to coast it rumbles much louder then quiets.... suddenly... as if it disengages something.

Well it's a mystery and I'm getting ready to tear it apart... or just slap it up front and run my new open yukon geared 4:88 THIRD in the rear.

Thoughts or suggestions?

 

Did you have the vibration before you changed the third member out ?, what about the axle bearings, check those as well because if there bad that can cause a vibration. I wouldn`t recomend a detroit for the front if its a daily driver.

 

No noise or vibrations before. But that was in a 4:10 third. So now far more rotations at high speed.

It's either
(1) bearings

(2) gear set

(3) detroit locker

Heard some stuff about the detroit acting funny by engaging and disengaging... so I downloaded the trouble shooting guide. Guess I gotta tear this thing apart and have a look at its guts.

 

Did you slide the driveshaft apart?? If you did, it could be "out of phase". The only time the locker will make a differance is when turning, or if you have tires not the same height. I love mine!!