Starter turns but engine will not fire

It's a 3.0 rwd XL.
I first noticed the voltage losing current, the gauge needle was "dancing" left to right, and the van it's self would sputter while this accrued, then would suddenly regain proper voltage and the problem would go away. This continued for a couple of weeks. Then one day while driving the freeway, I noticed an enormous cloud of smog trail from my van. I stopped for a while to allow it to "relax" then attempted to drive it home. About 20 miles down the road it made an extremely violent bang/clack (almost like an exhaust leak, but the largest loudest exhaust leak known to man) then it slowly came to a stop. I know it's automatic but still it smelled liked clutch. The engine wouldn't turn over but the starter obviously works no problem (it makes an obvious sound that clearly it's not turning the engine over) I had it towed to my house.
I've replaced the battery, starter, and the starter solenoid.
It sounds exactly the same as when I had the old installed so I know it's none of those 3.
(The cables seem to fine) but still can't find the issue......
Please help

 

put a socket on the crank bolt and try to turn the engine by hand.
sounds to me like you may have smoked the engine.

 

May have, but it could be a seized accessory or belt idler. I'd take the belt off of it, then try starting it. If it starts and runs, then shut it off within 30 seconds or so, as there is no coolant flow without the water pump turning.

Then turn each pulley/idler by hand in the direction that they should turn (grooved pulleys CW, smooth pulleys CCW as viewed from the front). If something belt-driven is seized bad enough to create that much drag, it should be obvious. (The smoke and smell was a possible clue there, could have been a stalled/badly slipping belt smoking)

If the engine won't turn by starter with belt off, time to try to turn crankshaft by wrench!