I finally got around to doing a quick wiggle test of the drive shaft. I've been getting this vibration that starts around 35-40 mph. I think it starts before that be that's when it sounds like there is loose change inside my dash. It feels like it's somewhere in the back end. The frequency increases as speed increases. First thought was U-joint but didn't really know where to start. New tires and alignment didn't make it go away. It used to go away at times...not during a drive but sometimes it would occur one day but not the next. Now it's all the time and the dash noises are just about enough to drive a person crazy let alone the rumble seat.

Now I didn't get a chance to get the truck off the ground yet, but crawled under last night for a quick wiggle test of the drive shaft and I don't see anything obvious at the u-joints, nothing moves at either U-joint with just muscle power. BUT when I wiggle the front end of the drive shaft I get a visible and audible movement of the shaft in the transfer case.

I want to check wheel bearings yet too, but this seems it might be the likely culprit to my vibration. And I need to figure out what part to order and how much time I need to do the job. Any confirmation?

Truck is a 98 5.4 4x4 Ext cab.

 

Change the rear dif fluid. When it breaks down, it will shake the truck a part. It won't only vibrate, it will start to shake bad eventually.

If it's the bearing, which I doubt, -then the seal would be compromised as well. You would see evidence of this leaking when inspecting.

Yea, you usually just need to change the fluid and use the right stuff w/friction modifier.

 

the front of your drive shaft ( the yoke going int o the T case) should not have movement like you described. I would be looking into that.

 

Wow I did not know that. Would it cause a slight vibration?

 

I can't recall working on any with slight vibration problems, but I imagine it could start out that way. By the time they were brought to me, -all 3 times that I recall, -they were really bad. Was the worst @ around 55 mph as it became difficult to keep between the lines. I recall one vehicle owner saying he kept driving it because he couldn't afford a new transmission. He didn't drive it on the expressway because it vibrated/shook that bad at higher speeds. He took back roads to work and back for about 8 months lol.

Anyway, it feels as tho it's coming from the trans, but it's actually the rear gears and ONLY the LS rear ends. They were all 5fours and all had 9.75 LS rear end.

 

Found the problem tonight at least the major one I think. It's the rear U-joint and it clear needs replacing. After jacking the back end up to release the tension it was easy to turn the driveshaft while the joint didn't move. So that will be first. I had changed the fluid some time ago (~25-35K maybe), but don't remember if it was the "right" stuff, but do remember adding modifier at the same time.

The front of the shaft still has that bit of lateral movement, maybe 1/32" or so and clearly changes the angle of the shaft. Will look into that after the U joint change.

While I had it jacked. I tried checking the wheel bearings and I got about a 1/32 or so in and out movement on the wheels by grabbing the tire and pushing and pulling straight in and out on both wheels. Nothing by push on one side while pulling on the other. Is this also a sign that I need to be looking at wheel bearings also?