I had to jump in here because I had the SAME exact issue with my 02 Buick Rendezvous except it was in the rear and it had air ride suspension...well it did at one point in it's life lol I never had it dealt with.
Anyways, When I looked at it on the lot everything was fine and dandy, brought it home and my husband seen that the passenger side rear spring was literally broken in 2 pieces...ended up being broken in 3 areas actually. I had the dealer I bought it from change the springs for me, but he refused to change the one that was not broken. I figured wth, at least the broken spring will be fixed.
I brought it home after having the spring replaced, and walked behind the Buick and my entire rear end was off. I had a very noticeable sag in the rear and it leaned towards the drivers side, like I said rear end only. I'm no mechanic lol but I was told by one that it was due to the old but not broken spring losing it's ability to hold the truck up? The new spring had a good firmness to it, and the older spring was losing firmness due to age, rust and so forth. I bought the Buick at 99k miles and only did oil changes. Hated it the day I drove it off the lot, but desperately needed something with a 3rd row seat.
Maybe if you change all your springs when you get your shocks/struts? done it would help? Other than that I have no idea why it would ride so unevenly. I drove an Envoy that had a slight uneven ride, but it was also a salvage with major frame issues. Anyways, good luck!!