didn't think it'd ever be said til today...
it's how i avoided a tow home.

my truck's a 99 RCSB sierra 4x4, and for the past few weeks, i've been having a bit of vibration while driving - mainly from 40-50mph but having a bit on both sides of that. i just put on some new D-range 33s in Dec and have heard that some people have had nightmares keeping them balanced (Falken Rocky Mountains/Wild Peaks), so i figured that's what it was, so wednesday i took them to the shop on base & rebalanced them. although the machine showed 3 of them zeroed out & one @ .25oz, when i left, the vibration was even worse than before. that's when i caved & started calling tire shops on saturday.

wallyworld was the only one with a lifetime deal & was cheapest, so i figured 'why not? if they screw it up, they'll just redo it for free'. wrong. when i left, it was just the same - so i went back & the senior tech (who actually did the balance) blamed it on my aftermarket wheels... the same wheels that i've been running for almost 3 yrs & with the last set of tires only balanced once and was smooth as butter up to 125mph. anyway, they gave me a refund and the number for another shop in town. took it there & the same deal - shaky as all get-out. called them when i got home & they said bring it back monday and a different guy would do it again.

that takes me to today. i'm driving down the road, rattling my brains out, when i feel a thump/clank followed by the worst shaking/vibration i've ever felt and manage to make out something bouncing behind me in the warbled image from the left mirror, so i pull over expecting to find separated treads or something & when i get out to inspect, everything still looks fine... so i crawled under the truck and noticed the left rear ain't got no weights nomore. that'd seem to explain the bouncing object. so i decide to putt down to the next exit and pull into a parking lot for further inspection. here i go, thumpity-thumping along for about 1/2mi down the ramp and up to the light but when i let the clutch out to go through, i heard a big clunk then clankity-clankity-clink as i quickly put the clutch back in. neutral seemed normal, so i left it there while i got out to check again. lo & behold, what do i see laying before me? my driveshaft has the diff end laying on the ground with shear marks on the pinion yoke. somehow, the u-joint caps had went bye-bye even with the clamps bolted down tight (i'm guessing that's what actually went flying in the mirror). either way, being a little mechanically-minded, i pulled the shaft off & threw it in the bed then hopped in and put my second floor shifter in 4hi and created a FWD truck to get myself out of the road.

aside from finicky clutch engagement and occasionally not liking going into 1st, it felt smooth enough to limp away. that said, i decided to not call AAA and babied it 5mi or so back to the house down the back roads. now's where i'm stumped...

i got it in the garage for inspection and can't make out what exactly went wrong. Dec 2011 i got a bit of speed-dependent clacking from the back end along with a band of vibration and i traced it to a sticky factory rear u-joint, so i pressed it out & replaced it with a new sealed unit from Advance last Jan. after that, it was business as usual. then the shaking started again a few weeks ago, and i was thinking the new tires as i mentioned above and didn't give a second thought to the driveshaft because not only was the u-joint brand new but it wasn't making any squeaks or clanks. upon inspection tonight, it moved smoothly/freely with my hand and had plenty of clean red grease where the missing caps were, so i don't see binding being the issue. that led me to think something might've bound inside the differential, so i jacked up the back end and gave the rear wheels a few turns. smooth operation, no noises. i turned each wheel individually in both directions and got the pinion yoke spinning accordingly as well as hand-cranking the pinion and getting the wheels turning in both directions. it's an open diff, so i even held each side while turning the pinion and got the other side to turn like it should. the only thing that ain't acting like it should is when i spin one wheel, the other is stationary as opposed to spinning the opposite direction (which i know it did yesterday at the shop).

that's as far as i've been able to get because it don't sound or feel like anything's broke, but then there's that one aspect that ain't working right, and it's got me stumped. what do y'all think or know?

 

morning bump 'cause i need to fix this today

 

sounds like a drive shaft balance issue. just because you replaced a u joint doesnt mean all is well. if you ask any drive shaft shop after they replace a u joint they will rebalance the whole assembly. if the driveshaft was so far out of balance it could have ripped the u joint right out of the straps on the yoke.

 

that's kinda what i started thinking after more digging last night. but what i don't get is why it would take a year for it to become a problem. if the new u-joint was out of balance, why would it've been so smooth this whole time? and it don't look like any of the balance weights are missing either.

and that also don't explain the lack of counter-rotation with the wheels free-spinning

 

could always pull the diff cover and check out the spider gears.

 

well apparently it needs the pinion to be stationary to get the counter-rotation... which makes sense thinking back on it.

i replaced the u-joint again and that must've been it because it drives fine now.

oh well...