Hey guys my '95 F150 Long Bed has a SkyJacker 4" Suspension Lift (Blocks In the Rear) and as of recently I've noticed some slight vibrations coming around 1500 RPM's. The truck has Cooper STT's that have about 10k on them they are all worn evenly I was just curious how big of a lift my driveshaft would be good up to, I've heard up to 6" Lifts. All U-Joints are tight and holding grease, I'm leaning more towards the tires on account of the vibrations were not there when I purchased the truck 2 years ago. Also the truck does sit in a warehouse for 6 months out of the year during winter. Any insight would be greatly appreciated!

 

Any ideas?

 

Are the blocks angled? If so are they backwards? Misaligned drive shaft to rear end?

 

Get an angle finder and measure your U-joint angles. Assuming no CV joint the angles should be the same at both ends of the driveshaft. A well-engineered lift should keep the angles the same, which means either making sure the rear axle doesn't rotate from it's original position, or else tipping the t.case down the same amount the rear axle is tipped up (but this messes with your fron U-joint angles).

Vibration is usually from the angles being different more than from the angles being too large. If you see a difference of more that a degree or so you probably need to shim your axle to even them out. If the angles are the same then the vibration probably is not from the lift.

 

Is the 4" lift new? If it is and the driveshaft(s) were removed the U joints may have been rotated when put back together. If you bought the truck with the lift already in place and your vibration is a recent phenomenon the issue is something else.
To me your issue sounds like a failing U joint. I have tested many U joints over the years and, unless the driveshaft can be turned easily by hand, it is very easy to miss. I would recheck them.

 

Does your truck have a center U-joint and a carrier bearing? (Not sure if long beds did or not…)