Here's a couple pictures of my FJ/5100 set up initial and currently. The first is FJ springs and 5100s set at 1.75. There was a slight reverse rake but it wasn't a deal breaker for me.
This picture is my current set up, same as before but I added a 1.5 spacer in the rear and Air Lift air bags to help when I tow my boat.
If the slight reverse rake is a no go for you then I'd definitely get a spacer or lift spring of some sort for the rear. Hope this helps. It's a great set up either way.
* just re read the title, if you are talking about using a 1.5 spacer with the front set at 1.75 I can't help you but maybe this will give you a starting point to go from.