I have a 2006 Chevrolet 3/4 ton heavy duty 4WD pickupQuestion:  While the truck is in motion, when I shift electronically from 2wd to 4wd the shift isn't smooth and it sounds like gears grinding, but it will shift into 4wd. Shifting back into 2wd is fine (makes sense). I suspect the problem is with the transfer case. I would appreciate any information from your experience what could cause this problem?Answer:    A few different problems can cause this to happen.  It could be just a worn mode syncro (inner) #701-1D , if that's the case you need part number 401701-1 your cost $33.93.  And mode syncro (middle) #701-3D , if that's the case you need part number 401701-1 your cost $42.92, mode syncro (outer) #701-5D , if that's the case you need part number 401701-5 your cost $50.900.Suggest you start by checking the fluid level and condition, the rear case has a tendency to wear a pin hole under the oil pump in the rear case resulting in a low oil level and damage to the mainshaft and drive hub creating premature synco wear also.  If the case has a whole you will need the following parts:  Item 760-2A Case Half Rear NP263 STD part number MWTMG620001 your cost $195.00.  Drive Hub / Bushing (splines into Drive Sprocket 2 piece syncro part number 401606 your cost is $48.98. Item 672D Main Shaft (49 Spline Pump) NP263HD, part number MWTNV48714 your cost is $199.89.Other items you need will be Gasket and Seal Kit part number TSK261 your cost $39.99 and Chain item 700E 1.5" wide 42 links part number MWTCL10-074 your cost is $144.40If you decide not to repair or overhaul you transfer case let us know we can provide a manufactured unit.  For more information on the parts we can provide for the NP263 see our website.