I recently upgrades my gears to 4.88 with an ARB in the rear. With the new gears I can finally get above 60 mph. I've run into a problem, when I'm above 55 and coasting there is a violent shaking in the truck. I've had the tires balance, but I probably need an alignment. I don't think its the alignment b/c there is no vibration under power.
Where should I look?

 

u-joints possibly.

 

ujoints x2

 

Im with the other guys! U-joints

 

I think I might add pinion bearings as a possibility too.

 

Mine does that at 70 and I was told the lower ball joint.

 

I've said it before and I will say it again...In my mind there is a big difference between a "high speed vibration" and a "violent shake".
To get any good advice on the web its important to mention the frequency of said vibration.
Your driveshaft spins almost 5 times as fast as the wheels now so it should be easy to distinguish between the two.
That would be a good place to start and it would narrow it down to a driveline issue or a wheel/tire ect issue.
Im going to guess that because you just regeared, it is a driveline(high frequency) problem. I would start at the u-joints then check the slip splines on the driveshaft. Remember, not only are you driving faster but the driveshaft is spinning faster than before at a given speed.
Sorry if Im talking to you like a nubie. Dont know how much you know.

 

It is a fairly high frequency shake, but its definitely a violent shake. The truck was literally shaking itself to pieces. I'm pretty sure its not a bearing the rear pumpkin was built with all new parts by trail-gear and front was done by a pro locally also with all new parts.
While I had things apart I took the time to actually lube the ujoints. They where completely dry. Probably a bad sign. They also look like they have been around for a while. For all I know they are original. What is appropriate play for a Ujoint?

 

A u-joint should have ZERO play in the direction of rotation.
Recheck the pinion nut too. If the nut is'nt punched into the keyway on the threaded part, it WILL back off. this would also explain why it only does it in coast.

 

have you seperated the driveshaft at all during the install?

If so...its possible the u-joints are NOT in phase