from the last few seasons?

The situation has changed since this entry was added.  As of late
1998, older episodes can be seen in syndication.

If you're curious as to why syndication behaved like that, Brian
Petersen, webmaster of The Simpsons Archive (http://www.snpp.com/),
filled me in.  What follows is essentially his explanation, with minor
editing for the current situation.

[Begin]
Syndicated episodes are sold on contracts that specifically specify
how many times each episode must run.  For instance, when the deal for
MWC's syndication was inked, it may have stated that each episode was
allowed 15 runs in national syndication tops.  Once that limit had
been reached for episode x, it can no longer be shown.  The reason the
newer shows [were] running "exclusively" is because they entered
syndication later than the newer ones, and [hadn't] yet reached their
maximum number of runs...

The end result is that you have a high concentration of new episodes
in syndication now, just like when syndication for MWC began, you had
a high concentration of old episodes, as that's all that [was]
available at that point in time.

Luckily, this doesn't mean old episodes are doomed.  Syndication is
done in two ways: on barter contracts, and on cash contracts.  Barter
is the "national" form.  The distributor decides what episodes air,
and all stations run the same shows on the same dates; kind of like
how it works on a network.  The rules that say how many times episodes
may air only apply to barter runs.  The reason it's called "barter" is
because the episodes are fed via satellite and contain a couple of
commercials that come from the distributor.  In return for airing
those commercials, stations get to show the series at almost NO COST.

However, some stations may opt to sign for a cash contract.  As the
name implies, they have to PAY for each airing.  The reason is that
this type of contract has no commercials coming from the distributor;
stations use an in-house tape library to play the episodes, and THEY
decide what episodes air: including old ones.  If your station has at
least one cash run, then they can just as easily decide to run old
episodes as newer ones.  They aren't under the syndicator's control,
after all.
[End]

Also, be aware that stations that show MWC in syndication are not
neccessarily Fox affiliates, they are simply stations that purchased
the rights to show MWC reruns in your area.