So I'm fairly new to the Blazer/Jimmy world so please forgive me.
looking at '99 Jimmy, fully loaded for my girlfriend. Guy says the transfer case is locked in 4 lo, and seems like he doesn't know too much about it. I have read and it looks like the vacuum, TCCM, or encoder motor problem. My real question is will the real fancy Snap-On scanner with the identity keys read the codes instead of grounding pin 16 in the diagnostic connector? I have access to a scanner and would rather use that because it will tell me so much more about other aspects of the vehicle. Also, its about a 2 hour ride to look at it, so I don't wanna make many trips if possible, so I'd like to get it right first time.
Thanks for all the help.

 

1998 and newer, transfer case codes must be retrieved using a scanner, your Snap on will do just fine.

 

I'm not certain about the NV233 transfer case, but I know for a fact that a high end scan tool capable of reading serial bus modules can read from the NV236 TCCM. We were able to read the TCCM on our 2001 ZR2 that has the autotrac 4x4. You cannot read the codes with a basic code reader though.

If it's locked in 4x4 it's likely a TCCM issue or an encoder motor on the transfer case, however it is possible that it's the selector switch IF it has the Autotrac 4x4 system. The automatic 4x4 switch uses a serial signal to select from auto 4x4, 2hi, 4hi, and 4lo. This ended up being the problem with my 2001 ZR2 where it would lock into one mode and would not switch without the battery post being pulled for 5 minutes.

If it only has 2wd, 4hi & 4lo it will almost certainly not be the switch, which means it's likely the TCCM or encoder motor. Check the wires and plug at the TCCM for corrosion. The TCCM is in the passenger kick panel. If it's the encoder motor I have no experience with that and someone else will have to help.