help with rear end shake

[eastcoastmat]

89 blazer 4inch lift blocks in rear factory 10 bolts 33inch tires, well guys im stumped!! truck drives smooth up till about 45-50mph then the rear end shakes or bounces horribly! tires balanced by two different shops, new shocks everything is tight. front end is perfect new u joints on rear shaft. cant figure it out. could worn out leaf springs cause this? there are no visible or obviuos problems. i have to drive this thing 1200 miles pulling a small trailer and have to figure this out. any input would help im burnt out trying to trouble shoot this,

 

[4xcrazy]

Make sure the drums have their little weights on them, or there aren't any clean spots where one may have come off.

If the tires & wheels have been balanced properly, it's something further in.

I know on our Avalanche, we get a shimmy type of bounce vibration out of the rear area, but I'm certain it's worn shock bushings, allowing a slight bit of movement at highway speeds. I stuffed wood shims between the shock eye and the bottom of the mount as a trial thing, tightened up that movement for awhile and the shaking went away, until they fell out

 

[eastcoastmat]

I suspect a bent axle shaft or flange. Local parts stores want 200 a piece. I found a online company with upgraded axle shafts for 89.00 a piece. Go figure. I hate to spend any money on these 10 bolts because I'm gonna do the 3/4 ton upgrade this winter but im moving to Kansas and need to drive the blazer. Oh well.

 

[4X4HIGH]

If you just installed the lift blocks and the issue arouse after doing so then you need to adjust your pinion angle.

 

[eastcoastmat]

Nope. Been lifted for years.

 

[4X4HIGH]

Who installed the new U-joints? The originals were held in by hot poured plastic and the replacement U-joints will require the use of internal C clips. If those are installed (make sure they are BOTH still there) then next check to see that the U-joint is centered in the driveshaft as someone here recently said they've seen some shafts that the insides of the Yoke weren't machine properly so the U-joint isn't centered even though the clips may be installed.

 

[eastcoastmat]

I did the u joints. I'll check to see. If its centered. There is a tiny bit of play inn the rear u joint .

 

[4X4HIGH]

I did the u joints. I'll check to see. If its centered. There is a tiny bit of play inn the rear u joint .

There should be NO play in the U-joint. Is the play a side to side and if so is it in the driveshaft or the diff Yoke? You may have gotten a wrong U-joint.

 

[eastcoastmat]

Yep side to side. The clips are in but there is still a little movement. If I turn the clips so the open ends are facing the rear theres quite a bit more movement. Dam auto zone. I still think the axle is bent. I took the tires off and put it up on jack stands then watched for movement. I could see it woobling.