jack up on rear tire, put the trans in neutral, and then rotate the tire around exactly twice while counting the number of times the driveshaft spins. 3 and a half will be 3.55, just over 4 will be 4.10, etc.
the link in my signature has a thread all about rear driveshaft lengths. that will help you find out if your new one will need to be longer or shorter than is is currently.
and if you're new to MJs, this thread is a gold mine