Hey guys, I just finished doing my 14bsf rear end swap last week in my '86 K5 Blazer. So far everything has been great driving around town, etc. Well today I decided to take a drive on the Freeway and everything was smooth as glass until I hit 70 mph then the truck started to shake very noticable. I when ahead and kept speeding up until 80 mph then let off the throttle. The instant I took my foot off the pedal the shaking stops, the second I put my foot back on the gas, shaking starts again. I can't tell where it is coming from, although I can't feel any of it through the steering wheel. I tried shifting out of overdrive into 3rd, no change.

The both driveshafts are stock, the rear driveshaft U-joints are good, the rear one is brand new (Napa 447), the front doesn't have any play and looks good and is about 8 months old. The tires are great (33x12.5x15"), although they haven't been balanced recently. The 14 bolt was a direct bolt-in from a 'Burb and is in great shape. The truck had a 4" lift on it when I bought it (no blocks). Shocks are the old Rancho RS5000 and are tired, but not dead by any means. Both the 700R-4 and NP208 have been rebuilt 2 years ago.

Sorry for the long post, but I didn't want to leave anything out.

Anyone have any ideas what this might be and/or how to fix it?

Thank you for your time guys.

its possible you're gettings vibes cuz your ujoints are out of phase from reinstalling the drive shaft.

its possible you're gettings vibes cuz your ujoints are out of phase from reinstalling the drive shaft.

Forgive me, but I am not sure what you mean. Do mean out of balance?

Thanks!!

well now that i think about it, that might not be your issue. your driveshaft probably slides in and out of your transfer case doesn't it? if it does this is not your issue.

basically its not lining up the orientation of the ujoint at either end of the driveshaft. but thats not possible to screw up with a slip-yoke shaft.(which is what i believe your blazer has)

not real sure what else it could be. i suppose its possible the rear end isn't set up properly, but thats beyond me to diagnose lol. you sure the ujoints are fully seated and tightened down properly?

Sounds like it could be either a driveshaft out of balance, maybe something with your new u joints, or after swapping that SF in, you somehow changed the driveline angle and now its vibrating cause the axle isn't shimmed to the appropriate degrees necessary for smooth operation. Are the tires and wheels the same ones that came off of the truck, maybe you lost a tire weight.

Correct, the driveshaft is a slip yoke type.

Sounds like it could be either a driveshaft out of balance, maybe something with your new u joints, or after swapping that SF in, you somehow changed the driveline angle and now its vibrating cause the axle isn't shimmed to the appropriate degrees necessary for smooth operation. Are the tires and wheels the same ones that came off of the truck, maybe you lost a tire weight.

I am going outside right now to check the tire weights. Same rims and tires as before. I never checked the driveline angle because I figured since I didn't have to relocate the spring pads and shock mounts everything would work.

You said something about 4 inches of lift on the truck, (too lazy to go back and reread) you didn't lift the truck with blocks or anything did you?
Re-check to make sure that your u joint bolt straps or u bolts or whatever you have holding the rear of the driveshaft to the diff are tight.

Tire weights are good.

You said something about 4 inches of lift on the truck, (too lazy to go back and reread) you didn't lift the truck with blocks or anything did you?
Re-check to make sure that your u joint bolt straps or u bolts or whatever you have holding the rear of the driveshaft to the diff are tight.

All suspension lift, no blocks. I bought new U-joint straps and bolts from the dealer and just checked them two seconds ago as per your suggestion.

Thanks for the input anyway dude!!

Tire weights are good.

You said something about 4 inches of lift on the truck, (too lazy to go back and reread) you didn't lift the truck with blocks or anything did you?
Re-check to make sure that your u joint bolt straps or u bolts or whatever you have holding the rear of the driveshaft to the diff are tight.

All suspension lift, no blocks. I bought new U-joint straps and bolts from the dealer and just checked them two seconds ago as per your suggestion.

Thanks for the input anyway dude!!

Forgive me, but I am not sure what you mean. Do mean out of balance?

Thanks!!

just so you know, out of balance is the normal weights missing/wrong weight/bent driveshaft, out of balance.

out of phase: your upper and lower yokes have to be in line. they can't be perpindicular. like this, looking down the drive shaft, upper yoke to lower yoke: -=>-----<=- . perpindicular yokes would be like -=>------=>-. this will cause it to vibe. the driveshaft is designed to balance itself when in phase. out of phase will cause a vibe. usually slip yoke driveshafts like yours are built in phase. a 2 piece driveshaft with a shaft in the middle can get out of phase easily and cause problems. they have fixed yokes on both ends.

just so you know, out of balance is the normal weights missing/wrong weight/bent driveshaft, out of balance.

out of phase: your upper and lower yokes have to be in line. they can't be perpindicular. like this, looking down the drive shaft, upper yoke to lower yoke: >-----< . perpindicular yokes would be like >------=. this will cause it to vibe. the driveshaft is designed to balance itself when in phase. out of phase will cause a vibe. usually slip yoke driveshafts like yours are built in phase. a 2 piece driveshaft with a shaft in the middle can get out of phase easily and cause problems. they have fixed yokes on both ends.

Yokes are inline, just like they are suppose to be.

Tomorrow I think I am going to put the whole truck on jackstands under the axles and run the truck through the gears until or if the shaking starts. At the sametime I'll see if the driveshaft woobles at all.

Read this :D

http://www.differentials.com/diagnose.html

Still think you should check to make sure that your driveline is set at the correct angle comparativly to your rear axle.
You know how lift blocks are tapered from the rear to the front? Well, they are that way so when you lift the truck using blocks, it keeps your pinion angle correct. If you truck was lifted with suspension only, maybe the pinion angle is not correct. You should check it and you might have to have it shimmed.