hear it in 4wd, was that because there was snow on the ground? Two reasons for this:
#1 if there is snow on the ground, that would change the tire noise, so it goes back to my first guess.
#2 if there wasn't snow on the ground, you have a part time 4wd system and you really shouldn't be driving it on the pavement at all in 4wd, it can bind up the driveshafts....unless the hubs are disconnected. If the hubs were disconnected, then you still had the front driveline and front axles spinning and that could cause a different sound from the front.
My guess is you had it in 4wd during wet/raining weather, that changes the sound profile, and it is indeed the tires.
What tires do you have and how many miles are on them?