Hi guys. I own a 85 GMC Suburban 3/4 ton two wheel drive. It has the 400 transmission and a 454 motor. It has a oscilating vibration at about 67 m.p.h. on up. I feel it in the floor and in the seat. I can run it up to 70 m.p.h. and take it out of gear and it stops. When stopped I can put the truck in park and get the r.p.m.'s up to where they were at 70 m.p.h. and there is no vibration. I can put the truck in 1st or 2nd and get the r.p.m.'s up to where they were at 70 m.p.h. and there is no vibration. I have put new U-joints and carrier bearing on it and it still has the vibration. I have looked at the drive shaft for a thrown weight and did not see anything. I put a new transmission cross member rubber mount on it. I also put a new fan clutch on it. I rotated the tires to see if the vibration changed location. It didnt. I've been trying to pay closer attention to where it is coming from and it feels like it may be toward the rear of the truck. What can I look for in the rearend.
I bought the truck like this thinking it was the U-joints so I don't know how long it has had the vibration. Man if you could help me out here Id be very thankful!! Mark
I have the same rythmic pulsating vibration that happens at highway speeds (no speedometer). Mine only started after I put a TH400/ NP241 and changed nothing else. It didn't have that vibe with the 700/NP208. My dads onl '79 2wd chevy with a TH400 did it too and my G/Fs folks '98 burban with a 350/700/NP208 does it too. I have yet find an answer for it. Some posible theories might lay into the torque converter weight problem or something don't know.
If you are uncertain about your drive shaft, or if you replaced u-joints, then you need to take the whole thing from one end to the other to a shaft balance shop that will spin balance the entire shaft as it runs on the truck. Cost here is about $45 and well worth it. This is recommended every time you even just install u-joints, although most don't do it.