Trailing Arm Uh Oh

[DaAbominableFlip]

So I'm driving to work the other day and this happens.
Now , what is weird is that the truck will roll in neutral and stay put when put in gear (engine off) , but will not move period if I step on the gas. Reason I put it in drive in this condition is because its about halfway in my driveway and I figured that scraping up a broken arm wouldn't be a biggy. I need a few pointers as to what is going on because everything on the rear end still looks to be straight. The frame is going through more hell than anything.