Alright, so I have this problem I have been trying to diagnose. On the highway at 65+ mph, it shakes. I have had both wheel bearings replaced, tires balanced multiple times, tranny just recently rebuilt, driveshaft straightened and balanced, new U-joints. Truck drives excellent at speeds up to this, but once 65 is hit, it begins to shake, continues to drive straight, but the whole truck shakes. I have put it up on jack stands and ran it up to this speed, the driveshaft would begin to shake near the rear diff. Is there something involving the rear diff that would cause it to do this at speeds above 65? If anyone has any more suggestions, or possible fixes. PLEASE HELP! haha

 

Its a 1999 chevy blazer, 4wd, 2dr, LS

 

Couple of questions, ok more than a couple. This shake you are trying to locate, where do you feel it the most? Through the seat, steering wheel, floor boards, through the whole truck or just from the rear? You ddin't state the transmission you have (5spd or auto) Why was the driveshaft straightened/ rebalanced? What size of tires do you have? what wheels are they mounted on? Have you tried rotating them to different positions?
I'll through a couple of ideas at you, that you may or may not have had.
To be honest, to me it almost sounds like a belt seperation in one of your tires. I have been down that road (so to speak) more than once. Have you looked at the backside of your wheels? Sometimes you can get a build up of mud on the backside of a rim that will cause a vibration. Also you mentioned that you have had the tires balanced multipletimes, why? and did the amount or position of the weights change? I know this sounds like a lot of tire questions but just trying to rule them out.
On to other things, when the driveshaft was removed was it marked for position relative to the yokes? It could be installed out of phase. How long after the transmission was rebuilt/ replaced did it take for the vibration( shake) to show up. Have you checked all your mounts ? Are they tight, broken, worn out etc. Have you installed a lift kit? If so how big? (drive line angles) What are they? Do you have any leaks at the pinion or transfercase yoke? Many times if a leak develops in one of those locations it's a pretty good sign of a bearing letting go. (shaft/ yoke wobble)
By the way running the vehicle up to speedon
jack stands and watching the drive shaft probably won't reveal much do to harmonics on an unloaded chassis.
Just some things to ponder.

Good luck,
Gator

 

Not to hijack the tread, but my 2000 blaze does the same thing... At around 65 mph it shakes like I threw a weight...

1st I rotated the tires, no help
2nd replaced the tires and re-balanced, no help

Maybe I have a rim out of round, but since I rotated the tires, the shake still comes from the front end... I feel it mostly in the front at the dash board....

 

Do you guys think that an alignment is in order to try and fix this?

 

check your bushings and ball joints when you go and take it to get an alignment done they will point that out first if there bad and steering or suspension components that are bad they more than likely will tell you to get them fixed first before an alignment.

 

jkinn22- Just wondering if you figured out what was causing your shaking?!? I think I'm going to take mine into the shop to get it looked at.... Not looking forward to that BILL...

 

Vibrations.
https://blazerforum.com/m_25271/tm.htm

Read it, just becuase the tires are balanced doesn't mean they wont cause vibrations. Are the tires Road Forced Balanced? Read up on it in the above link.

 

Awesome! that post helped clarify the balancing of the tires!! Here is my question... On that link you provided (where you could look up tire places with road balancing machines), One of those locations just put my new tires on for me ( I bought BF Good A/T from them and they did the mounting, and balancing)... Would they definitely balance my tires with the road balancing machine? I've had the shaking problem since before I purchased these new tires... About 6 months... Then I got new tires and the shaking didn't go away...

 

I cannot say what thier policy is, however in the shop I worked at it went something like this. You used the nearest balancer. If you had trouble balancing a tire you drug everything down to teh Road Force machine. If a customer came back complaining of a vibration, you checked them on the Road Force machine. We offered a 90 day warrenty on all tires we mounted and balanced. Plus you could buy the extended lifetime rotate and balance package, then every other oil change bring them back in for rotate and balance at no charge. There were a few guys that provided excorts service for the big rigs (not the lady of the evening escorts). They were in every month for a rotate and balance, free of charge.

First thing is take it back to them and complain of a vibration. Ask them for the road force numbers.