There are two different parts depending upon which model and car you have. If you have the New Yorker (Salon?) (C-body short wheelbase version) - you should have the standard suspension. However, if you happen to have a New Yorker Fifth Ave (Y body -long wheelbase version) you could have either the standard suspension or the air suspension. The air suspension (used on some Fifth Aves and Imperials) does not refer to the option of air height leveling shocks but rather the use of air bags. 1991 Mopar parts numbers are listed below - which are now no longer available.
4228 653 CROSSMEMBER, Rr. Susp. Control Arm & Axle Channel w/Std.
Susp.
or
4421 255 w/Air Suspension
You have a couple of options. You can go to junkyard and get a replacement one. If you can't find one locally go to Harry's U-pull it in Harzletown, PA. But you have to remove it yourself.
http://www.wegotused.com/If just the lower spring perch is rusted out or almost rusted out (which is very common) - you can buy a replacement part and weld it on or take it to a shop and have it welded. I had one replaced on my 93 Chrysler New Yorker. Links below to comparny, online store and catalog.
http://autorust.com/http://autorust.com/store/http://autorust.com/2008cat.pdfPart Number : ART-65