Are the cover bolts loose? Mine used to loosen up and seep/leak on my 2000.
I finally fixed my diff cover on my 2000 (3rd gen) a couple of years ago. I was able to get the cover off and on by removing the thru bolt of the front motor mount and jacking up the front of the engine to "roll" the engine/transmission back (at the top) to give more room between the cover and the aluminum cradle. I had also dropped my pan and filter first to get rid of any fluid that would keep spilling into the diff area.
I cleaned all the old silicone from the case and the cover, degreased and all. I used ultra black rtv silicone to seal it on. I washed all the bolts in gasoline, dried them/let air dry and wrapped each one 1 1/2 times with white teflon tape to act as thread locker. Put it all together and snugged the bolts down, and let it sit overnight. Did the same with the trans pan the next day, but I used the rubber gasket and adhesive instead of silicone. Refilled with new ATF+4 and it hasn't leaked a drop ever since.
I know the 4th gen van has emissions stuff down in that area, so that will likely get in the way. Hopefully my setup technique still gives you some room to work with. I used a 10mm combination wrench, and 1/4" drive sockets and ratchet. Best of luck!