The rear crossmember where the lower control arms/toe control arms bolt to IS flimsy.. Stiffening up those points makes a big enough difference to notice.
and an upper is really needed with the eclipse hatchback The avenger has the rear deck which really negates the need for the rear upper. but the rear lower makes a big difference IMO in both vehicals
Im not saying its absolutely needed to get an eclipse to handle decently, but obviously its going to improve it.
