shake from rear 55+ mph

Anyone have any idea why my 98 blazer would have a shake and shimmy from the rear at highway speeds? Sometimes it's not there other times it shakes the whole vehicle, usually it's right in the middle though. I've tried shifting to N to see if it was from the driveshaft but it was still there when in N. I don't think it's an out of balance tire but I'm going to rotate them this weekend to rule that one out. Any one have any other ideas as to what may be causing this?

I'd say driveshaft and / or U-joints. Get under the truck and grab the driveshaft and u-joints. If there's any play or movement ... not good ...

tranny mounts ?






Anyone have any idea why my 98 blazer would have a shake and shimmy from the rear at highway speeds? Sometimes it's not there other times it shakes the whole vehicle, usually it's right in the middle though. I've tried shifting to N to see if it was from the driveshaft but it was still there when in N. I don't think it's an out of balance tire but I'm going to rotate them this weekend to rule that one out. Any one have any other ideas as to what may be causing this?

ill bet its the front u joint

Shake or shimmie or a hop at 55-65 MPH range points to tire and wheel balance.
To test jack it up and put stands under rear axle.
With tires and wheels on crank it up to 55-65 mph and check for shake and the remove tires and wheels and bolt drum with lug nuts and crank it up to 55-65 MPH.
If it still shakes you have a drive line problem.
Then if no shake while the tires and wheels are off get the tires and wheels balanced and tires checked for a slipped belt.
Let us know how it goes.
MT

They are all correct but if not it then mine was caused by the rear disc/drum assembly. The drum section for the parking brake was slightly oval and the disc was perfectly straight. I jacked the rear up and put jack stands under, removed wheels/tires, put the lug nuts back on to secure the drum/disc, started it and run the rpms up and saw the out of round. Be careful and a two person job - make sure vehicle is properly held on the jacks and pointed out away from crashing into anything if a mishap. New rear drum/discs at $35 each corrected - I chased this thru the entire wheel/tire, u-joints, diffs, torsion bar mounts, shocks, etc - not common but easy to fit once identified.

Can't beleive noone said to check for clumps of mud inside of wheels, for me seems to be the stickiest place for mud, also the hardest to get out, and easiest to get there.

One more idea, anyone ever see how violent a tire gets when you have a bad shock? It's looks like a basketball on a low dribble at higher speeds. Mine where bad and you really couldn't see much of a leak. Replaced all four since they had more than 100K on 'em.