[RobbyMartin]

I have a 7.5" Fabtech lift on my truck with a 5" block in the rear. Before the lift I could drive it at any speed without the vibration. Since the lift and the 4.56 gears I have a vibration at about 80 mph. I've had the gears replaced once because of a faulty ring and it still does it. Is this going to be the norm or is it something that can be corrected? Maybe the u-joint or out of balance driveline? Any thoughts?

[fordkindaguy]

My best guess is that you probably have off road tires and they are not balanced correctly. With larger tires this is common.

[88Racing]

Originally Posted by fordkindaguy

My best guess is that you probably have off road tires and they are not balanced correctly. With larger tires this is common.

Good answer!

Also this Is the pinion angle set at the proper pitch with the use of shims or tapered block? If this isn't done you will start to go through ujoints also.

I have seen tapered blocks put in backwards an upside down along with shim wedges forgotten.

Just some ideas!

Lars

[RobbyMartin]

It isn't a tire issue because its not that kind vibration. The blocks are tapered and are impossible to install upside down because of a pin and hole in the top and bottom of each block. I'm going to replace the u-joint and see if that helps.

[theSLEEPER]

Do you have a carrier bearing? If so, you might want to drop it down a little. If not, you might want to see if you can space the transfer case down a bit. I don't like getting too much tilt on the rear diff...

[fordkindaguy]

make sure to replace your yokes while you do the differential!

[Power Hungry]

Pinion angle, especially with a carrier bearing, can be a continuous cause of annoyance with lifted trucks. If it's not vibrating at highway speed, it's shuddering on hard acceleration. It is an absolute pain to get everything lined up correctly.

One thing we've had to do is build a spacer block to drop the carrier bearing a bit. Of course, this throws the front yoke off a little and may require dropping the tailshaft of the trans (via modified crossmember) to get everything back in line.

Lifts are a pain. It's just easier to cut 6" out of the fenders!!