No guarantee, but as far as I know, about 90% of the EGR-related failures
around this vintage are due to the DPFE sensor. It's fairly easy to test
with a vacuum pump and a scanner. In a pinch a crude DPFE test can even be
done with nothing more than a voltmeter and some 'smart' messing with the
throttle (engine runnig), forcing the computer to open the EGR valve.
Of course, it would have been cheaper (not to mention faster) to pay someone
with the proper equipment and knowledge to diagnose, but I do understand the
pleasures of throwing parts at at a problem -- have done it myself, more
than once. So since you have already replaced just about anything else, you
might try the DPFE now. But before you do, make sure that the tubes leading
to it are not clogged or reversed. Good luck!

By the way, I vaguely remember an 'extended warranty' or something like that
for these failure-prone DPFE's. Won't hurt to check if your '96 is still
covered.