Rear-End/Whole truck shimmy/shake

[RangerRyan88]

Alright guys, I need some help I am really confused. First off, what I'm working with- 1996 Mazda B3000 Ext. cab 2WD 2" leveling spacers on the front. Haven't touched the rear-end. I parked my truck about 2 days ago, and everything was fine. No shakes, rattles, or shimmies.

Today I get in my truck to run some errands and my whole truck shakes, feels like it's coming from the rear end. It parallels vehicle speed, and I can notice it as slow as 10MPH and gets progressively stronger. I'm too scared to take it on the freeway to see if it stops.

I searched around here, and guessing from a few posts and the wetness (leak) on the bottom of my diff cover it could mean my diff is going out? I just find it really weird that this started after sitting for 2 days, not after a hard trail ride or anything.. Also I haven't EVER wheeled my truck hard at all so I really strongly doubt it's a bent driveshaft or axle shaft..

And I don't know if this might have been my imagination, but I could have sworn it felt like the rear axle "steered" around a couple corners.. Maybe loose U-bolts??

Sorry for the long post, and thanks in advance for any help you can give me!