vibration around 40-55 mph in my 96 f-150

i have a 96 f-150 4x4. it is lifted and runs on 33's.  truck has been fine since i got it 2 years ago.  about a year ago i had a u joint going bad in the rear so i replaced them both.  everything was fine till about 3 months ago when i noticed a vibration. it is not constant.  it will vibrate for about 2 seconds then quit for about 5 seconds and then vibrate again.  it doesnt do it pulling out, just when there is no real strain on the truck.  it doesnt do it freewheeling or braking.  seems to be coming from middle or rear of truck.  i rotated tires..didnt help.  i checked u joints ...there good.  i noticed my clutch pack was wore so i put a mini spool in an open carrier i had and put it in today... still vibrating.  could it be driveline ot torque converter...oh i also drove it with overdrive off and it still does it.   HELP PLEASE  

Better check U-joints again as it sure sounds like them. If it was drive shaft balance it would do it coasting or not. Ujoints can cause different vibrations at different power levels.  

I would agree that it sounds like U-joint  
lets go wheelin

thanx alot i will check them in a few hours and i will let you know if that is the problem.  

I would beat that if you took one out you would find wear on trunnions. (the place were roller bearing in caps ride)  

i have checked the u joints they are snug.  any other ideas?  maybe torque, pinion bearings....im lost here!!!  

Did you pull the caps and look at them? I have little doubt that the problem is in them. They can feel snug and still have a bad/dry cap.  

i doubt that they are dry because i grease everything on my truck about every 2 months but i will check that out the next dry and warm day i get.  thanks for all your help.  

Consider this, I have a old J20 Jeep truck that I have owned for 22 years now and when I used to drive it daily I had to replace Ujoint in front drive shaft about every year. It has full time 4wd and drive shaft is not very large for clearance issue and runs at a bit of a angle even stock. After a years time I would start to get a vibration in front drive shaft even though I greased it regularly with high quality grease and they still felt tight. When Ujoint where changed it was good for another year or so. (I actually had a extra drive shaft so I could have a rebuilt one on shelve) When you lift a truck or had bigger tire and do not regear you place a higher average torque load on Ujoints and it can and will shorten their life span.  

Check the center carrier bearing on the rear driveshaft if your truck has one. This can cause many of the symptoms you describe.  

i had a friend ride with me today and he said that the clutch works off of static or something.  he said it was the fluid.  he suggests that i change the fluid and filter in the tranny and put some friction modifier in it also.  does this sound like it may need a looking at?  any help will be greatly appreciated.  also if i get a warm day 2morrow im taking out the u joints and checking them.  

last post said "clutch works off of static"  i meant tranny works off of static.  i was on phone with buddy talking about his clutch in his mustang while i was typing so disregard the word clutch.  

How about you just change the u-joints and the chances of it going away is pretty good. My truck was doing the same vibe thing at about the same speed maybe a bit more. I just put new Spicer joints in it along with the new shaft in less than 20,000 miles ago and sure enough there was just enough movement from side to side and up and down that I decided it was time for new joints. Good luck.  

i just changed both the u-joints and took it down the road....no change what so ever.   could it be in the torque, pinoin bearings, driveline out of balance or anything else.    PLEASE HELP...THANKS  

How old are your tires? Ive had tires go bad that caused vibration and only after replacing my u-joints did i even suspect the tires. Also, Is your driveline in phase? A common mistake when replacing the u-joints is reinstalling the drive shaft out of phase with the yokes which can cause vibration. And another thing, check that your u-bolts on the axle are tight. if your truck is lifted, loose u-bolts especially with lift blocks, could cause the axle to rotate to an angle where u get vibration.

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