A YJ's front shaft does not spin when driving, so the only fuel savings would be weight.
UNLESS, he has swapped to a one piece passenger side axle shaft (eliminating the vacuum disconnect, there would be a plate over it) OR he installed a lunchbox locker.
I mention the above because it may not have been removed because of fuel mileage, but maybe vibrations (balance, angle, etc).
Call any local driveshaft shop and they probably have some in stock. Can you post a picture of your front axle and the shaft? Want to make sure it is the D30 and a DC shaft or stock shaft.
93' YJ 2.5L, 4.88's, 8.8, E-Locker & LSD