Van will not start after replacing the muffler

Did you accidentally knock a wire loose to the fuel pump or something?
Can you hear the fuel pump run when you first turn the key to run, before you turn the key to start, for about two seconds?

How did you remove and install the replacement? Welding or clamps?
Did you have the battery disconnected? You say it will not start, does it click or just turn over?

Okay, just trying to rule out a blown ECM if a welder spiked it. It is rare but does happen if the ground is not positioned on the pipe. A bad O2 sensor will not create a no start as it is not used during a cold start during open loop operation. Poor running and setting a code once the PCM expects data from it as it tries to go into closed loop operation will happen.
No start is either fuel, ignition or internal engine related.
A short cut test would be to spray a bit of starting fluid into the engine and see if it fires on it. If it does then you should have spark and the engine internals are likely okay, but you do not have proper fuel. If it does nothing then the likely suspect would be a loss of spark. Or you could pull a wire off of one of the coil packs and check there for spark with a spark tester. Those engines do have a bad habit of the ignition control module failing.

I would also check up near the engine itself. One of the crankshaft sensor harnesses is just below and between the exhaust outlet on the manifold and the pipe, It could have been pulled free/broken if you were that close.