GMC Repair: transfer case?, rear drive shaft, p1870 code

Question
i have a 1999 gmc sierra 1500 5.3L, 4-whl drive,z71.i don't use the 4-whl drive.it has 123,000 miles. i don't drive rough. when starting out or slowing down, it makes a clunk noise & jerks.sometimes in the front & sometimes in the back.but then it smooths out.there was an 1870 code.Trans. mechanic said it could be transfer case or transmission.he said it wasn't doing it excessively enough for him to tell. it only does it every 2 or 3 weeks.but i don't want to break down. any advise?

Answer
Hi
As far as the clunk/jerking, I would, WITHOUT actually seeing and feeling vehicle, tell you that SOME noise and "looseness" is normal. GM has a bulletin on this.
You MAY also want to have ANOTHER mechanic check the REAR drive shaft input shaft and have it lubed. There is special lubricant (grease) to put on splines.
The P1870 is an code for Transmission Component slipping. The Transfer Case CANNOT set this code.
I would tell you to take it to a transmission shop and have it diagnosed. The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply.
•  DTC P1870 is a type B code. The conditions for setting the P1870 DTC must occur on TWO CONSECUTIVE TRIPS (ignition cycles, with a drive cycle) before setting a P1870 history code.

  •  When the conditions for setting DTC P1870 are met (first trip), the PCM commands maximum line pressure and harsh 1-2 shifts are the result.  

  •  This may result in a harsh 1-2 shift with no history code if the conditions for setting the DTC required for the second trip are not met, on two consecutive trips (Ignition cycles, with a drive cycle).

  •  When the conditions for setting the DTC are met, on the second consecutive trip, a DTC P1870 is stored as a history code.

  •  When the P1870 code is stored, the PCM will turn on the Service Engine Soon (SES), Check Engine Light (CEL), or Malfunction Indicator Lamp (MIL).  

I would HIGHLY advice you to have it properly diagnosed.
I Hope this helps.
Good Luck
Pawl