Yes, the big nut on the back side of the fan needs to come off. Since most of us don't have a wrench that size, there are different "procedures" we have used. First off, it's a left hand thread, meaning you push down on the drivers side to loosen it. Many people use a punch and hammer, tapping down on the edge. That didn't work for me, I used a vice-grip and a short 2X4. Clamp the vice-grip on, not too tight, place the end of the 2X4 against it, and whack it with a hammer. Remember down on the drivers side. If the belts are already off, someone will need to hold the fan, or jam it somehow.
When you put it back on, just get it snug, the engine torque will do the rest.
'96 2500, CTD, 4X4 flatbed, Isspro gauges, Skyjacker D-25's, ThurenFab track bar, DSS, HD steering.
Motor; BHAF, #100 fuel plate, 5X.012 90hp injectors, Pacbrake 4kGSK, Loose starwheel. 306hp/743tq before injectors.
Exhaust; Schwitzer S300G, 4" downpipe, single 5" turnout stack.
Trans; Dunrite VB/TC, Intelligent Engineering billet input shaft, Burt Brown rebuild/upgrade.