Have you done the calibration procedure for the TPS? Have you cleaned out the IAS? Are all the Vacuum lines for the PCV system intact? Is the Vacuum system for the 4wd system intact and not leaking? These are problems I've had on my 1989 Larado and my dads 1990 Larado that gave hi or surging idles.
Tcases would be either:
Command Trac: found behind all 5psds and lower end autos. Is a 3 simple position case. 2WD, 4WD high, N, and 4Low, Ok so technically there are 4 positions but who counts "N"?
Select Trac: Found only on autos. So 2wd, 4wd Full Time(aka AWD), 4wd Part Time, N and 4 Low.
Unless someone has been swapping things you should have the Command Trac.
Fan is controlled along with the AC compressor. AC will come on when you are in defrost and AC positions.