Rear end feels like breaking loose and regaining traction?

This is with Hoop-Tay, the 2010 Crown Vic Police Interceptor.  So by no means is the car actually breaking traction. 

At anything above about 40MPH if i push the loud pedal, when I let off I get the barest hint the rear end us giving just a little shimmy.

It reminds of breaking traction and then regaining it, or last time I had a tire with low pressure in the rear it acted the same.   A little gas, weight shifts and you get some suspension squat, you let off and the suspension unloads and realigns.

Doesn't feel dangerous, but it does feel like there is something a little loose.  Never had a car with a panhard setup before so I'm not sure where to start poking, or even if it needs poking.   May just be inherent to this suspension design. 

Edit: not Panhard, Watts Link

Big, squishy bushings.

Yea, I'm guessing something in the lower control arms is worse on one side and letting the axle do a little steering trick on you.

I figured as much.

I really need to get this thing on a lift before the challenge 

Does it have a Panhard? I thought those were Watts.

Every solid axle 4.6 Mustang Cobra that I drove had a torque pull.  It was weird.  Not sure if it was a limited slip fighting tires of slightly different diameters or what.

If it has a limited slip I'd run a tape measure around the rear tires to check for stagger.  Then take the wheels off and make sure both upper control arms are still attached to something.  What you describe is also what it feels like when Trailblazers would rust an upper arm off, although those would be more like automatic lane change function.

Pete. (l33t FS) said:

If it has a limited slip I'd run a tape measure around the rear tires to check for stagger.  Then take the wheels off and make sure both upper control arms are still attached to something.  What you describe is also what it feels like when Trailblazers would rust an upper arm off, although those would be more like automatic lane change function.

Swap the wheels side-to-side and see if it pulls the other way?

The torque tries to turn the differential, lifting one wheel and pushing the other one harder into the ground.  Even if there's not enough torque to spin one of them, it will still alter the rolling radius slightly.

Pete. (l33t FS) said:

Does it have a Panhard? I thought those were Watts.

It may technically be Watts.  I've seen different things claimed with the same level of authority and haven't gone digging.

I do know when I was under it I didn't see a bell crank centered on the rear axle, it had a bar running from the top of the diff to the side of the frame, fairly sure thats Panhard

No limited slip, yet

Hrm.  Looks like they put the bell crank in front of the differential, not behind.  Weird.

Streetwiseguy said:

Big, squishy bushings.

Or, if your car comes from a location where roads are salted in the winter, a rusty control arm that's getting ready to break.

In reply to stuart in mn :

From what I can tell, control arms are all aluminum, actually.  Not that it matters here, car is a former Bradenton Fl P.D. vehicle

Yep those 8.8 Watts links are a pretty wild design with the linkage on the front of the diff.

How many miles? All the bushings might be worn out.

In reply to maschinenbau :

170k-ish

Unfortunately, I don't think that *cough* pig *cough* will make it through my shop back to the lift.

In reply to jgrewe :

Dont worry, man.  I didn't even consider it.  LOL.

All rear lug nuts and axle/wheel bearing stuff tight?

but the real answer is bushings, probably.

Assuming nothing is just flat broken, I'd look at bushings AND axle windup. Those axles twist quite a bit. Big bar/big springs could help in that department.