if one corner in the rear is sagging , it can be because several things ,not all just rear end related..
it can be that one of the front coils is for instance not totally in the groove its supposed to be in...makes it 'taller' , and result is the rear corner on the diagonal will be pushed down = looks like sagging, ( did you make sure the coil got in correctly?)
also the subframe could be bent , same result , diagonally it gets pushed down if frame is bent up, or vice versa..

if frame bent, you can use coil spring adjusters ( too types , one that twists in to raise or another type that can raise or clamp dn, you`d want the latter)to clamp down the side thats too tall up front...to get car even , not a perfect solution ,but a solution..

other suggestions were ,broken leaf, bad bushing etc,
if the springs are good , you can get them re-arched(did that on another car), you can also add a leaf into the exist set , very inexpensive(I did that to lift the rear on my 57 Stude 1")

on my bird I bought a new set , got the 'original' style 4 leaf...5 leaf is cheaper ,but somewhat stiffer....depending on how you want the ride to be, softer ,more orig.= 4 leaf , stiffer ,more 'racy' = 5 leaf

may want to check the front....even though sag is in the rear...