Periodic hum/vibration under car every ~3 seconds

Getting this weird low frequency hum and vibration felt through gas pedal and floorboard while driving at around 30mph. Lasts <1 second and happens every 3 seconds. It's more pronounced while coasting but that might just be the lack of engine noise and vibrations. Also more pronounced when the truck is fully loaded up.

Thought it was the U joints at first since there is sometimes that clunk when shifting to drive or reverse but I don't feel any play in them when I had it up on jacks.

Any ideas??

 

Do you ever lube the driveshaft zerks?

 

Not yet, don't have a grease gun yet. I noticed above the u joint behind the transfer case there's either grease/oil flung off onto the underside of the truck so either the previous owner greased it and the excess got flung around or it's been leaking

 

Might want to check Tcase fluid level

 

I'd grease everything and check the front wheelbearings. My rearshaft would clunk when coming to a stop, it was dry. Grease made the sound go away.

 

Yep, and rear end as well.