I have to agree with the folks who say it is county/city related.
First, I wave at any cj, yj, tj, and even jk that passes. If an MJ, XJ, or SJ is appropriately modified, I'll wave at that too.
Here in Jersey, I have experience in 3 counties:
When I lived in Hunterdon, I got 95% wave return from the above vehicles (JK's didn't exist then).
When I lived in Sussex I got 95% return from all but females driving JKs. There it was about 20%.
Now I live in North/East Somerset on the border of Morris, where I get about 75% return.
For whatever reason, in Long Hill township, and on the road through the great swamp, I get 100% return, even includinjg JKs.