Hey all, I've already searched for this one and can't find any help.

I have a stock 96 Bronco w/ 68K miles and a 351. At speeds of 55+, when I let off the gas I get a clunk and a feeling and sound of driving on a rumble strip. Obviously the first thing I tried was replacing all the U-joints and rebuilding the double cardan joint. It seemed like it might have helped but its definitly still there. What else could this be? Pinion nut loose? Need new diff fluid? Any one ever have this same issue and know a fix?

 

well it hard to say but the pinion nut backing off is very comon. DW

 

would that cause those symptoms? All I have to do is drop the shaft and tighten the nut?

 

yep but you need a big pull handle it torks to min. 160ft. lbs DW

 

cool, thanks for the tip, I'll try this ASAP.

 

it wouldnt hurt to check the pinion nut, and while you got the drive shaft loose you may as well grease the slip yoke. DW

 

oh yeah, forgot to mention, already did that too. Why would a loose pinion nut only cause vibration when letting off the gas?

 

I knowyou already said that you replaced the u-joints, but double check em and make sure that the yokes are lined up correctly from one shaft section to the next. The factory balances them with the yokes lined up. For details on what I mean, try this site and look up driveline replacements. %

 

are you sure the vibration is coming from the rear. It could also be your front end, how does your steering feel. Jack up one tire at a time and see if there is any play, could need bearings repacked.

good luck

 

just greasing the slip yoke may not help ... check it's condition, if there is excessive play in it it's worn and most likely the cause of your problem. the sound is your driveshaft flopping around, the clunk is when it finally straightens out (caused by binding) this will eventually take out your rear output seal, pinion seal (and possibly the bearings - causing your pinion to appear loose and 'floppy')

 

Slip yoke felt pretty tight - its a 96 with only 60K miles - but I'll check it again this weekend.

 

check with the trany in nuetral and all tension relieved from the shaft. another problem area that will produced similar results is the CV - most people are unaware that there is another lube point between the two u-Joints in the CV, you'll need a needle type grease fitting to grease it up (don't know if you knew about it or not) if the center ball and stud run dry too long the whoe CV will become "sloppy" and create excessive vibration.
Just don't want other to have to endure the torment I did trying to find the problem (due to previous owners poor maintenance).

 

Are you referring to the ball within the double cardan joint? I replaced that too

 

Just an update and a thanks for the suggestions, I dropped the shaft again today and tightened the pinion nut. Got at least one full rotation. Voila! Problem solved. No more clunking when letting of the gas and the "rumble strip" vibration is completely gone. Its nice to find an occasional problem you can actually solve!

 

Good deal, chalk it up! another problem solved without a $65.00hr shop! DW