Use a caliper and measure your own sway bar. I made this mistake a year ago and now have to redo the job with the proper size bushings. There's unfortunately no sure fire way to determine the correct size here, if you didn't own the vehicle since it was new, someone may have even swapped the bar with a different size.
Since its RWD that rules out your CV axles. Double check wheel bearings again, even back ones if fronts are good. Noises are sometimes strange in these vehicles. If all that is ok, check for looseness of any brake pads front and back. Check tightness of all steering components, inspect tie rod ends, inner tie rods, and the rack for any signs of play, damage, or corrosion. You noted your upper ball joints are rotted, this needs to be addressed as if let go it could be a serious safety issue. I recommend getting the entire upper control arm assembly from MOOG. I replaced both mine in under 2 1/2 hours with just hand tools, you should have no problems.
Once you fix those if the issue is still present you may want to start looking at your drivetrain for play. Check the drive shaft, as well as play in any of the four tires. Grab them at 9 and 3 and then 12 and 6. While off the ground and supported by a jack AND stands that is. -This should show you if there's any wear in any suspension components. If all else fails take it to the shop and have them diagnose it. Unfortunately noises can be very difficult to identify as they could be 1 of any 1000 parts.
