The limitation is in the IFS front drive design. The CVs have X degree of upward and downward movement, regardless of where the diff is sitting. You can add the diff spacers, which moves the front carrier housing downward, which allows you to use a larger spacer, but you will still have to contend with moving your upper strut mount down (or make artificially longer) to compensate for the drop of the diff.
When you add the 1.5" spacers, when at rest your CVs will be angled down. They aren't "maxed out" in their downward movement, but since they are angled down you have lost some of the downward movement designed into the CV joint. If you add a 2" spacer, your CVs will be more severely angled downward at rest and have virtually no more downward stroke available to them. For example when you are driving on trails and drop one wheel into a hole. The CVs will be constantly operating in a severely flexed position which will cause premature CV failure. On the bright side you would have quite bit of upward movement available to you.
You can virtually "extend" your strut by a using a spacer which Jeff and Mudd Child offer, or you could use a strut extender which basically screws onto the upper end of your stock strut to make it "longer". Which ever you opt for make sure the spacer or extender equals the spacer you use on the spring. If it is too long or too short you will either bottom out or top out your strut which will either cause it to fail prematurely or will grenade on you on hard compression or catching air when wheeling (ask me how I know). The best made extenders (IMHO) are the ones Dave has made and offers for sale on his Bits4Vits web site and on e-bay. His have a small set screw machined into the side which allow you to lock the extender and prevent it from loosening when you are driving.
Another design limitation on these trucks is the rack and pinion steering. The more you space the springs... the more of an angle the front tie rod ends operate in. At some point the seals in the R&P rack will start seeping.
The manufacturers made a major design trade off on modern light duty "trucks" (versus earlier trucks) which was to move away from the straight axle design in favor of the "smoother ride" IFS offers. There are always trade offs. I think the straight axles designs were more beefy and more reliable.
