if you haven't popped the cover in a long while, that would be my choice. refrshing the gear oil and checking for anomolies is a good thing.
if you have checked it not too long ago, then you can do the quick and dirty ratio check by chocking the front wheels, jacking up one rear tire, put the trans and t-case in neutral, then rotate the tire around exactly twice while counting the times the driveshaft spins. if the driveshaft spins about 3 and a half times, that's a 3.55 ratio. 3 times is 3.07. a tick over 4 is 4.10s. etc.