78 Chevy 4x4 still bucking at 33-35 mph, help?

Hey all,
I've been posting off and on here whenever i get pissed off enough about my truck to see if anyone can help. It's a 78 1/2 ton 4x4. At one precise
speed, approximately 33-35 mph, the thing starts to buck up and down. It's not a side to side motion, it's honestly what you would call a 'gallop'. It's been doing this for over a year and i've done everything i can think of to stop it. the transmission was rebuilt about a year ago, had no effect, the axles have been swapped with new gears put in, new calipers, pads, rotors turned, i've had 2 sets of wheels and 3 sets of tires all mixed and matched on there at some point in time with no effect at all. I had one end of the driveshaft replaced cause it was showing a little wear. I have no damn clue what is causing this vibration. I have asked every gearhead and mechanic i know and no one has any ideas beyond a tire being out of round or rotors needing to be turned (which i have already tried). Any input or advice would be greatly appreciated.
Thanks again TB
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assuming it is an automatic, i am wondering if it is trying to shift gears. i have a little 4 cyl s10 and if i get in the right speed/load it will start to buck as it trys to shift up and down. i see this on the tach and a little gas takes it over the edge and it stops. also, maybe a u-joint...

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Would it do it accelerating and braking? The gallop happens during both
thanks TB

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How about removing the rear drive shaft and running in 4wd this would eliminate or confirm a rear shaft problem you will need a dummy yoke to stop oil leaks, how about running it on a lift at 35 mph, see if a helper can spot any drive train movement or excess runout.

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I tried that, still vibrating, so i ruled out the rear shaft. I've put the rear axle on jack stands and you can still feel a vague vibration, not near as bad as driving it though.
Thanks TB

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Look at the trans mount and engine mounts for collapsed or broken insulators.

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On Sun, 27 Mar 2005 18:16:44 -0600, "Trailblazer"

Coasting down through 35 mph with the engine shut off (or towed)? Have you tried a different gear? 2wd, 4wd?
-- Regards Gordie
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I was able to try it in neutral today, it did the exact same thing. An odd note, it gets worse the warmer the truck is. I live in northeast AL and today it's about 55 degrees and it didn't start until about 5-10 min of driving it. yesterday, it was about 65-70 degrees and it was worse the first time i drove it. I tried it in 4wd and 2wd back when i took out the rear driveshaft and drove it, it did the same thing, same vibration at the same speed. Gear wise, it's an automatic, and if you accelerate hard through that speed, it's not as noticeable,but it's still there.
thanks TB
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My old '78 blazer had similar issues, but the gallop was at higher speeds ~50mpg. (6" lift with 35" tires) Mine turned out to be the mechanical fan and/or clutch assembly. I removed the fan, went with elecrtic and the gallop went away. It's a long shot but I am sure you're willing to try anything at this point.
HTH,
Derek
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Do you think it might be that the rear brake soft line may have imploded

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I don't think the rear line is bad, but would it cause that vibration at one speed? I also forgot to mention that for about 6 months the brakes seemed awful, like they wouldn't stop the truck hardly. Come to find out, the booster was bad. But what seemed odd was that during the booster not working, the gallop seemed to not bee there. I don't know if it actually stopped or if i was just holding my breath hoping i would be able to stop and wasn't paying attention for the gallop.
Thanks! TB

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On Mon, 28 Mar 2005 13:23:00 -0600, Trailblazer wrote:

Well that pretty much eliminates the drive train. How about suspension?
coldfire
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suspension's factory rear leafs with 2 in. blocks, and replacement front springs for 2 inches of lift all around. It has the dual shocks on the front end, i replaced 2 of them a year or so ago, the other fronts seemed ok, and i replaced the rear shocks about 2 months ago. All the bushings are polyurethane and we greased them well before putting the lift on, which has been on there about 2 1/2 years.
Thanks TB

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you might want to try looking at a misfire in the engine.I would start at the egr valve-these usually give problems in high gear under light load-try disconnecting vacuum line.
good luck
--
Message posted via http://www.carkb.com

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My 84 K5 Blazer did the same thing as yours. I though it was the calipers and asked a friend, he said it may be but might also be the brake lines. I replaced my front calipers and then the lines. The bucking went away with the replacement. My bucking would have shaken your teeth out of your head dentures or not.
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