Ronald while I too suspect the welded diff is the root cause you have several things going on with the set up. First what condition does it understeer? Corner Entry? Mid Corner Exit? Under Acceleration?? Steady throttle?

First there is the welded Diff, then you're using 60 series fronts (stiff sidewall) in combination with 80 series rears (tall soft side wall). Also you put stiffer springs in the front while leaving the stockers in the rear which will also changing the balance toward understeer. Finally while you put heavier oil in the front dampers if it can't control the stiffer spring the front tires will be oscillating up and down (read not on the ground)

Put the open diff back in, get a matched set of tires, if the rear isn't lowered to match the front correct it and then see what it does.

Tom