You could always take a very clean dust cap with the hole, put a
flathead bolt (on the inside) and a nut on the outside then goop it
up with JB Weld or even silicone to seal it...

Or try and find the original plastic cap to plug the hole if anyone
has them....

http://www.bus-boys.com/frontaxlepartsvan2wd.html  (they don't have them)