We have several ISI racks on different vehicles. They all use the ISI threaded pin / inserted flange system which works great. Basically the "pin" is replaced with a fully threaded bolt (we use M16 x 2, 130mm although 120 mm might work -depends on your receiver), then a flange consisting of an M16 x 2 nut welded to a square of flat stock is inserted in the hollow bar of the rack. Slide the bar & rack into your receiver, slip the flange in, line up the holes, thread the "pin" (bolt) through the holes in receiver/bar/flange/bar/receiver and tighten. The bolt pulls the flange against the bar and the bar against the receiver tightening everything up nicely. Bar/receiver wobble gone.
However, there can be wobble that isn't induced by bar/receiver slop. That often comes from loose clearances in other components of the rack system - especially if the rack is a drop down or folder style. If you remove the bar/receiver slop and still have more wobble than you like you can try using a line to secure the outboard ends of the wheel trays to the bumper so the trays don't "seesaw" around the single point suspension of the bar/receiver combo.
Howard