Really when exhaust system parts start going bad the best thing to do is to
replace the entire system (except for the cat if it is in good shape).  This
is because usually all the exhaust system components are the same age so if
some are going bad you can almost bet the rest are on thier way too.  The
other reason for doing this is because when a part of the system develops a
leak it takes the pressure off of the rest of the system.  When you put the
new components on this places increased pressure on the older parts of the
system therefore making them fail quicker.
Having said all of this, you may not have enough money to replace an entire
syncro exhaust system as they are very expensive.  However I sell a nice,
aftermarket exhaust system that is made by S&S Headers for the Syncro
Vanagon.  This complete system with muffler is only $280 (cat not included in
this price) and it is supposed to give you a horse power increase (one of my
customers that installed this system said he is experiencing a power boost,
though I don't know how great).
Good luck with your van, just wanted to let you know that there is an
alternative to buying a high priced exhaust system or just doing a partial
pipe replacement.
Ken Wilford
Van-Again
John 3:16