Ok, I ordered front and rear Coast driveshafts around christmas time. I put the front one on right away because my stock front shaft was toast. Got the new one on, adjusted the pinion angle the best I could, and still be able to drive it. All is good.
I didn't get around to installing the rear shaft until Memorial day weekend. Ever since then I have been battling a viberation from 55 to about 65mph. It's not a bad viberation by any means. You can hear it more than you can feel it. I have adjusted my pinion angle anywhere from 2 degrees less than the driveshaft, to 1 1/2 degrees more than the driveshaft. I cannot get rid of this viberation. I have had my tires rotated, and re-balanced. That didn't make a difference. If I remove the rear shaft and drive it in 4wd, the viberation is gone. The only thing I can figure is that even though it's new, this shaft is out of balance. Would you agree, or is this just the nature of the beast with these aftermarket shafts? I could definately live with it the way it is, but I'm so freaked out about blowing up my transfer case, that I'm a little hesitant to drive it.
It sucks If this shaft is out of balance, because the closest driveshaft shop is an hour away from me.
I haven't contacted Coast about it yet, because I don't want to start accusing them of anything, and come to find out I'm the idiot for installing it wrong.

 

I am running both front and rear coast drive shafts with no vibration. I would take a close look at all the u-joint caps and lock rings to see if they are properly seated. There has been some issues with the lock rings not properly engaged in the grooves and allowing the caps to push out. It is always possible a shaft got by that may have some imbalance. If you can not find anything wrong visually you could take it to a local drive shaft shop and have the balance checked.

 

x2, Check that the flange bolts are torqued correctly and that the bolts are all in good condition. replacement bolts can be obtained from any Ford dealer ( there used on mustang's) . Make sure the DS is greased and check U-joints. if these things are OK. check the rotational balance of the shafts.

don't hesitate calling Coast, they can help answer your questions and deal with your problem. they've been in business for a long time.

 

even though you already did, try to get a shop to see your pinon angle and have them set it up good. after my d/s install, i had a slight shimmy around 20-40 mph and at highway speeds... after driving it around this way initially, waiting for an appointment with the shop, the car started to make noise when off acceleration. Found out that my pinon angle was off, pinon nut was backing out and the seal was leaking... simple mistake became a BIG expense. now that I have my rear upper adjustable arms with all the items fixed, it drives very good. Also, thank the lord that my gears didn't get f'd up since I took it in right when the car was having issues...

 

Not to get off subject but what size lift are you running?

 

i'm running RK2.5 max lift kit. I haven't personally measured the lift but people on 2dr were getting close to 3.5-4 inch lift.

 

Wow. Hope I'm not having the same issue. I'm running 4" and have developed a weird squeak myself. Also noticed my pumpkin is starting to get wet. Might have to check this out. I assumed it was the DS.