Did you ever call Icon and question them? Keep in mind, just because they offer a "KIT" it doesn't mean it's "complete"., I think this is misleading and many places sell incomplete kits to lift the 80. Many here already pointed you to what is likely wrong. Panhards were not adjusted likely causing bump steer, I did my lift in stages 20 years ago and I seem to recall this, if you look from the rear you will see your axles offset to opposite sides. If your not sure what this is google it, sound like your issue?
If they replaced the radius arms of castor correction did they replace the bushings at the frame mount as well? It's well documented here castor correction bushings are not the best way to go as well. several choices to do it right and get the correct radius arms for the lift. You could go on and on with a 20 YO vehicle.
Other things that "kit" does not offer you will likely need:
Radius arms with new oem bushings, your bushings seem to have got you correct caster but likely will cause other issues down the road so likely short term solution.
Rear adjustable upper and lower arms with oem bushings
Possible DC drive
Rear panhard relocation bracket or adjustable panhard
Front adjustable panhard
LSPV adjustment if Icon did not have this with install instructions.
List goes on and on. but these are major items and minimum for a lift outside wear and tear/ maintenance needs for a 20 YO big fat pig. Especially if you are not original owner and don't know where it's been.
You chose a mechanic that does not specialize in lifted vehicles, they likely did it right but probably don't have the knowledge to understand what all is needed to make it right. Not their fault, they are probably focused on making good money on what they know.
Tons of info here if you search, you co go on for days as it's hard to find everything with a simple search. Quick search I found this. Most is bickering and opinion but if you read through it and use common sense it gets you pointed in the right direction, sometimes lol...
forum.ih8mud.com
HTH, not an expert here by any means...