[luckyguess]

is check engine light on?
if so find out why?
if not jack the van up and unbolt the exhaust before the converter, there should be a flange before the cat converter.
once converter is not in use start the van and see if if the it is still sluggish..
the popping and loping should stop if the converter is clogged.
it will be loud without the converter and muffler but it will breath and let you know if the exhaust is clogged and kicking backpressure back into the motor which will cause your symtoms..
if it runs the same then hook exhaust back up and move on from there...