GMC Repair: Harsh 1 to 2 upshift, chevy silverado, fuel starvation

Question
I own since new a 1999 Chevy Silverado 1/2 ton SWB 4x4 with a 5.3L V8 and O.D. automatic. Trans is experiencing harsh 1 to 2 upshifts intermittently (hard enough to ring the driveshaft) during unloaded or low load conditions while starting off from a stop. Presently has 106k miles, fresh oil and filter 10k ago by dealer ( only 7k miles towing 3,500 pounds max with tow/haul and little 4x4 use. New electronic 2W-4Wheel switch 2 years ago at 70k. At this time, the computer was also upgraded in 8 areas where current software was replace. 3 addressed drivability and #16264245 was replaced by #16264212 for 'new calibration to correct harsh 1-2 upshift' although I hadn't noticed any systems at that time. Is this the current upgrade to deal with harsh 1-2 shifts or if this problem isn't a electrical control issue, could you give me an idea of what mechanical problem might be causing this? Anything I might check in addition to oil level which is OK? Appreciate your time.

Answer
Hi
Although I am not a great transmission tech, I will assist some I hope!
First thing to check, unless done, is fuel filter. Fuel starvation can create numerous drivability concerns. including harsh shift points.
Was the PCM reprogrammed 10k ago or 2 years 70k?
That is an old program, but I cannot look up Cal ID's without your 17 digit VIN. Thats your choice to send it to me, you can click "private" tab before sending it to me and I'll be the only one to be able to see it.
Here is a list of harsh 1-2 shifts MECHANICAL or hydraulic causes

Checks
          Causes

Valve Body Assembly (60)
   •  Mislocated valve body to spacer plate checkball or checkballs.  

  •  1-2 Shift valve train stuck due to sediment  

  •  Gaskets or spacer plate incorrect, mispositioned or damaged  

  •  1-2 Accumulator valve stuck or damaged  

  •  Face not flat  

  •  4-3 sequence valve stuck or damaged  

  •  #1 or #8 checkball missing or mis-located  

  •  1-2 accumulator valve bushing rotated 180°  

2-4 Servo Assembly (13-28)
   •  Apply pin too long or too short  

  •  2nd servo apply piston seal missing, cut or damaged  

  •  Restricted or missing oil passages  

  •  Servo bore in case damaged  

2nd Accumulator (55-57, 104)
   •  Porosity in 1-2 accumulator cover or piston  

  •  Piston seal or groove damaged  

  •  Nicks or burrs in 1-2 accumulator housing  

  •  Missing or restricted oil passage  

  •  1-2 accumulator piston spring not seated  

  •  Rough finish in 1-2 accumulator bore in case  

  •  A cracked 1-2 accumulator piston - allowing fluid to leak by  

2-4 Band (602)
Worn or mispositioned

Oil Pump Assembly (4) or Case (103)
Faces not flat


I personally have found the 1-2 accumulator having issues and also the Valve body sticking valves. ANY debris in the tranny can cause valves to become sticky. I have run across a few checkballs being stuck in the spacer plate (#1 I think because #8 is in the case side).
If you want to send the VIN I;ll look up the latest and greatest updated software.
I'll await your response
Thank you...
I Hope this helps.
Follow post back private if sending VIN.
Pawl