I see this has been a few days, so hopefully you're still in need of an answer. Or at least an opinion.
In my experience, you can sometimes get away with up to 2", but often as not, could barely get to 1.5" without needing a drop.
Maybe depends on options and condition. I think there were at least two different types of driveshafts used back then, and some were more sensitive than others. At least that's what my feeble memory is telling me.
When we were lifting YJ's all the time, we got vibrations at least 70% of the time with simple add-a-leaf installs.
So we pretty much recommended a T-case drop for any lift over 1".
If you're doing it yourself, you can certainly try to get away without one, but you might as well buy one up front and have it handy just in case. Return it if you don't need it, but it's there if you do.