Rear suspension help

[Lated]

I recently lowered my 70 c10 4" in the back (and an unknown amount in the front but that's a story for another day). I've put new tires all around and had the front end aligned since I lowered it. The truck tracks fine going straight but when I go around a right turn of decent length (like an on ramp) at about 35 miles an hour the back end feels like it floats around the corner. I say floats because it feels just like the rear is sliding but the tires aren't losing their grip but it definetly isn't tracking with the front. I've only noticed it turning to the right but I can't remember making any similar turns to the left so it could do it in both directions. I installed an adjustable panhard bar when I lowered it and I thought I had it centered but now I'm not sure. Could the panhard bar being out of adjustment be causing this, and if so what is the best way to measure to make sure it is centered under the truck. Any other ideas of what might be making this happen? I have a rear sway bar that I want to put on but I want to get this fixed first. (This pic reminds me I still need to get the correct length bolts for my diff cover so I can get rid of the extra washers)

Dave