75 Blazer Rear End Clunk??

[old75blazer]

At least that is the best way to descibe it. My truck has developed a noticable clunk in the rear when stopping and starting from stop. When the brakes are applied and the truck comes to a complete stop there is a noticable clunk (somthing shifts underneath). Once the truck is given gas and starts to move clunk again (somthings shifts). I looked underneath while a friend drove and saw some movment in the diferential. I tightened all the U-bolts on the rear end and check the spring bolts too. Any I Ideas?

'75 Chev Blazer
4" lift (rear blocks)
Polybushings on the springs.
350 w/turbo 350 trans NP 203 t-case converted to part time.
new u joints on rear the drive shaft

Checked the rear end, normal ammount of play for chev, took cover off everything seemed ok.
It does the same in 4 H L and or loc.

Update: last weeked we completed a rear shackle flip on the truck. The problem is completely gone now. After all the replies and some work I think it was a combination of poor pinion angle and the slip yoke. The shackle flip rotated the pinion up to about the stock location. The flip provided approx 6" of lift in the rear. This was a semi home made flip kit, we used spring hangers from sky manufacturing and the stock mounts. I'll do seperate post on that.
Again thanks to all for your help and input

 

[79k20350]

did you pull the cover? gov-bomb?

 

[dremu]

Driveshaft may not be slipping, and when you stop and start it wants to move a bit. There's a grease zerk on the slip joint; lube the heck out of it.

Could also be U-joints on the driveshaft; check each end for play.

-- A

 

[ZooMad75]

I'd suspect the ujoints. Block the wheels and put the trans in neutral. Then try to rotate the driveshaft by hand. Since the wheels are on the ground it shouldn't move if the joints are good. If a joint is bad it will clunk back and forth when you move it.

If the trans is still in park or gear (manual) the driveline will have the wieght of the truck loaded on it through the wheels, even if the joint is bad, it won't move. You must check in neutral. Just chock the wheels so it don't run you over when underneath....

 

[dremu]

Just chock the wheels so it don't run you over when underneath....

I categorically ever deny having done this before...

-- A

 

[roadnotca]

Your specs say "new U-joints", any connection? Open dif. or? 30 years old, could it be the crushable (I'm no expert on that one)? Mine has developed a clunk after a 3 day off-road trip, so I'll be following along.

 

[79k20350]

I'd suspect the ujoints. Block the wheels and put the trans in neutral. Then try to rotate the driveshaft by hand. Since the wheels are on the ground it shouldn't move if the joints are good. If a joint is bad it will clunk back and forth when you move it.

If the trans is still in park or gear (manual) the driveline will have the wieght of the truck loaded on it through the wheels, even if the joint is bad, it won't move. You must check in neutral. Just chock the wheels so it don't run you over when underneath....

not true. the drive shaft shouldnt move but the whole assembly will. because of the backlash in the gear setit will move some. you have to rotate the shaft slowly till the gears mesh theen if theres any more play its the joints.

 

[marv_springer]

My old 76 did this for a long time... Never really found the root cause - but it went away when I installed the Doubler..

Mine had the same drivetrain too...

Marv

 

[79k20350]

hmm i wonder if its the differential in the 203? does it do it in 4 lock hi or low?

 

[ZooMad75]

not true. the drive shaft shouldnt move but the whole assembly will. because of the backlash in the gear setit will move some. you have to rotate the shaft slowly till the gears mesh theen if theres any more play its the joints.

You are right...I should have noted that. I just know when my front joint on the rear shaft in mine was gone I could rotate the shaft 10* in either direction. Plus you feel/watch the slop in the joint itself by pushing up. It was pretty obvious on mine that it was the u-joint and not anything in the rear end.

I fixed my problem by replacing the joint and adding a shackle flip to the rear springs. took away the drivline angle issue I had before.

 

[Leper]

I also say ujoint