Bad vibes

If it feels that violent, I'd definitely be looking at the driveshaft/ujoint. Not the same, but I have experienced what you're talking about in a cherokee after I dropped a new engine in. It'd bang around like I was missing a motor mount. Turns out some of the bellhousing bolts had come loose and it was allowing the engine/trans to sag. After you'd drive it a bit, it'd settle down as I'm guessing the trans would "align" itself. I wouldn't have easily caught except it finally struck the crank sensor which shut it down completely. Once I saw the gnarled sensor, I crawled underneath and noticed the 1/2" gap at the bottom of the trans/engine (well bottom side). Once I retightened it went away.

Not sure if that could be your issue, but it's easy to check.