Rear end Bouncing
 

I got this 1969 GMC truck and i was looking on here and couldnt find any problems quite the same as mine. Its a 3/4 ton, and i attempted to get home from college one day, and i was going down the road and i heard a loud pop and then the rear end of the truck started bouncing, at about 45 miles an hour. I turned around and went back. I blew out my hanger bearing after working on the drive shaft for about 24+ hours, Aka rust and stuff. I dont really dare to go over 40 again due to the chance of it happening again because when i get up to about 40 the motor seems to sudder for a few turns which seems like a carb problem and the rear end of the truck starts to bounce. Its not a smile bounce its like one i had to stop completely before it would stop. Any advice, Should i replace the rear leafs or what? Just looking for some Advice!
Thanks Robert

Re: Rear end Bouncing
 

This might be a long shot. A rear wheel may have thrown a balance weight, and the unbalanced tire makes the back of the truck shake. I had it happen once and it can make a loud bang if the weight hits inside a fenderwell.

Re: Rear end Bouncing
 

Your saying you blew out a drive shaft hanger bearing? did you inspect the U Joints and replace them? Could the bounce be a tire that threw off a balancing weight? Rear leafs got nothing to do with it as long as they are not broken.

Re: Rear end Bouncing
 

Maybe a shock collapsed or broke off a mounting point.

Re: Rear end Bouncing
 

I would lean toward something being wrong between the the tail shaft of the tranny and the pinion of the axle. Drive shaft balancing weight is possible. I once had a violent shake on my 4Runner at speed and it turned out to be the nut retaining the yoke to the transmission output shaft had loosened up. Never had another issue after just re-torquing it. It could be a separated tire but it would be more continuous throughout all speeds, same with a wheel weight. As for it being a balancing weight, it would have to have been a big ass weight to shake as violently as the OP stated. Try rotating your tires front to rear to see if it migrates. That will eliminate tires/wheels.

Re: Rear end Bouncing
 

Anything hitting the all steel bottoms of our trucks makes a loud bang, such as the chunk of front brake drum I lost on the C-20! Just a thought, OP should check those rear drums.

Re: Rear end Bouncing
 

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Might also be a tire tread separated from the casing.

LockDoc

Re: Rear end Bouncing
 

I will give all that a look, I did have to replace the Drive shaft yokes. One of them busted, took about 3 hours to get it out cause all the rust... These trucks are so much fun when they have been sitting in a field for 15+ years Haha

Re: Rear end Bouncing
 

When my hanger bearing went out once it started jumping it wouldn't stop until the truck was completely stopped.

I could speed up until I hit a small bump then it would start jumping. Yeah it's an unnerving jump like you describe. Time for a new hanger bearing. If I'm understanding correctly what you've done so far.

Re: Rear end Bouncing
 

Been driving it a little more and now its not bouncing as much only at high end revs does it start to stutter and then starts bouncing, I think i might have a timing problem or an intake problem. The motor will start missing, and then I push the clutch in and slow down it a bit and it catches back on. By the way i got a 2500 so its got a 3 speed and it happens mostly when i go over 50 mph. I had a buddy give me the part that controls the dwell, Its at home and i'm up at school right now but any thoughts on this would be great!

Re: Rear end Bouncing
 

How are the rear shocks? is it a leaf or a coil rear? I see it is a 4 eyes. I have seen many of the older trucks [remembering my youth]driven by older men around here the trucks were passed down farm truck maybe to a farm hand or a hand's family member but anyway the were retired from hard service the rear shock broke off in most cases those trucks hit a bump and would bounce until the truck would stop at the store or whatever. It was so funny to follow one just bouncing up and down in the back like they did. Of course I was driving a leaf spring truck by then and never understood why they did it until I was a little older and gained some understanding. Jim