I'm Gettin' Some Bad Vibrations...c

[chefduane]

98 4.0L SOHC 4x4 XLT, 5r55e recently rebuilt, 192k miles.
Ok so... for about the past 6 weeks now I have been getting a real low amplitude high frequency vibration developing and its getting worse. It kinda' sounds and feels like a real low hummmmmm that starts at about 40 mph and increases as speed increases. At 70 I have to back off as its really scary.
This has been developing BEFORE I had the trans rebuilt... I knew the trans was already in bad shape and a rebuild was in the cards, but I thought that perhaps the rebuild might fix the vibration problem. No joy. I have done a tire rotation/balance. It seems to be coming from where the engine and trans mate. It occurs whether I am in regular 2x, 4x, or 4x Low, and ONLY under power. If I am at speed (say 60 mph) and I shift into neutral the vibration goes away. If I rev while coasting at speed there is no vibration. So it has to be non-engine, non-trans, drivetrain related, eh? I am thinking front driveshaft u-joint.. maybe both front and rear? Possibly transfer case?
Driveshaft lost a balance weight? It's not coming from the rear of the vehicle as far as I can tell so I am tentatively ruling out any differential issues. Also, I take meticulous care of this vehicle.. all fluids, etc. changed at proper intervals, regular component maintenance/upkeep, etc. I have done a visual of the undercarriage and cannot see anything out of the ordinary. No fluid leaks or any fluids being thrown around. Nothing seems to be loose although I have not really given the driveshaft a good twist to see if there is a problem there. I do not want to have a catastrophic failure at some point so I really need to get to the bottom of this issue.
Any input from anyone??
Thanks!!

 

[masospaghetti]

I had a similar symptom as you describe in two vehicles - first was a 96 Camaro, second was my 96 Explorer. The Camaro actually had this happen twice, first was bad axle bearings, second was bad diff carrier bearings. The Explorer had a driveshaft that was slightly out of balance or slightly bent.

Both of these were only speed dependent though, it didn't matter if power was being applied or not.

Since yours is power-sensitive, you are probably on the right track with the u-joints or the front CV shaft.

 

[chefduane]

I'm going to start with F and R U-joints. RA has them for about $25 a piece. Not sure if I can do them myself as I have never done them. Local shop quoted me $220 for both installed. CV half shafts are about $50 each (R+L for a 4x4). Those I am sure I would NOT try myself. There is a shop here in town that will do work with carry-in parts so I may go that route.
Thanks for the info.

 

[SoNic67]

My front shaft CV was doing that... I replaced on both prop shafts the U-Joints and the one CV (AWD here).
They are not complicated but you need a vise and probably a helping body (rust might fight you).

 

[chefduane]

I may reevaluate doing the cv halfshafts if needed. Some of the videos here and utoob seem fairly straight forward. I have all the tools. However, there is some discrepancy on the torque spec of the axel nut. I've seen 150lbs, 165lbs and 170lbs. Anyone know (or know where I can find) the torque spec for the axel nut?

 

[chefduane]

SOLVED! U-joints. Did front and rear u-joints and now all is nice and smooth! Front U was pretty much rusted out on one lobe, all needle bearings were gone and the thing was just flopping around. The rear was not so bad but still needed replacing. Glad I got them both done and everything is a-ok. Also, I am REALLY glad I don't have to do a CV shaft job!!

 

[SoNic67]

I am glad that replacing U-Joints worked out for you! Is good that you did all of them, after all, they were all the same age and mileage.