I chased a similar problem around my car for 20,000 miles, periodically spraying one kind of lube or another at all the rubber bushings. Sometimes the squeak would go away for awhile, sometimes not.
I think SP is onto the right idea but in your case the problem may be at the other end of the car.
When I pulled my rear suspension apart last winter to upgrade the axles, I found the Rose joints in the upper arms were dry and binding. I adjusted out the binding condition as per the SCOF tech article and lubed the joints with some moly-containing machinery lube. I've driven the car 3,000+ miles since then and haven't heard the squeak.
I also replaced the sway bar bushings with poly bushings, so that may have been the problem, although I always gave them a shot of spray lube whenever I sprayed the suspension bushings and it never seemed to help for long. I think when the squeak did go away for awhile it may very well have been because I inadvertently got some
oil on the Rose joint.
Please let us know what you find.
Lowell