The top nut and grease fitting doesn't come off, at least I've never pulled them off. Just unscrew the shaft, clean all the old grease and grime out, replace the bushing, put the shaft back in pre-loaded with some grease (helps if the great fitting is out) then grease using the fitting. The idler arm design is really not so great IMHO as grease has to go through the threads of the shaft, and I always said the next one I re-did I would drill and tap a hole in the side of the bracket for a second grease fitting.
The 2WDs and the 79-80 4WDs do have the same shaft and bracket. The arm itself is different though if I recall.
_________________
95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good