My transfer case went out so I put a np 241c in its place now I can only get first and second by manual shift

It's a 2000 GMC Sierra z71 originally had a no 246 in it

I know this is way old, but I figure if I was looking for almost the same information and came across this real quick, maybe someone else will find this useful. GM uses a VSS signal to determine your speed. VSS is mounted on the end of the transmission in 2wd applications and on the output of the transfer case in 4wd applications. In 4wd applications, when you're in 4L, your output shaft is spinning 2.71x slower than the transmission output, thus if you're driving at 25km/h, your ECU is only seeing 9km/h. To rectify this, the t-case also has a 4L switch to tell the ECU it's in 4L and the ECU simply multiplies the VSS signal by 2.72 when the switch is grounded. This gives your ECU and trans the correct VSS signal/speed. Summation: If you have a 4x4 application, your VSS is in the t-case output. You must either supply a 4L signal (Green ECU connector C2, Pin 16 to ground) so the ECU will convert the VSS signal to the correct speed, or relocate your VSS to the transmission output (there are lots of ways to do this).

Durp. 2.72:1. Also, if you're moving at 25km/h in 4L, your ECU thinks you're doing 9km/h. 9km/h isn't high enough to shift out of first to second. That's why you're having issues.