any combination of bushings in the back, probably started squeaking when the new shocks raised the vehicle slightly upsetting the "set" the bushings took on.

It's a sign you may be approaching new bushing time. Check the sway bar mounts, etc look closely to see how many look squished out.

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)