263 hd transfer case for 2002 chevy 2500 need a diagram

https://tnttransfercase.com/general-motors-transfer-cases/

I guess you probably don't know what transfer case you have ?
SERVICE 4WD
Refer to
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Transfer Case Description and Operation in Transfer Case - NVG 149-NP3

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Transfer Case Description and Operation in Transfer Case - NVG 246-NP8

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Transfer Case Description and Operation in Transfer Case - NVG 261-NP2

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Transfer Case Description and Operation in Transfer Case - NVG 263-NP1

The NVG 246 EAU provides 5 modes, Auto 4WD, 4HI, 4LO, 2HI and Neutral.
The four manual mode, or range gear positions, of the NVG 261 transfer case are:
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2HI - 2 wheel drive high range

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4HI - 4 wheel drive high range, part-time

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4LO - 4 wheel drive low range, 2.72:1 gear ratio reduction

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N - Neutral, 4 wheel

The NVG 263 transfer case provides the driver with 4 manual mode/gear positions:
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2HI - 2 Wheel Drive high range

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4HI - 4 Wheel Drive high range

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4LO - 4 Wheel Drive low range

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Neutral

With the info I've given you ,you should be able too now !
There is probably a code stored somewhere !
DTC B0790 DTC B2725 DTC C0300 DTC C0305 DTC C0306 DTC C0321 DTC C0327 DTC C0359 DTC C0374 DTC C0379 DTC C0550.

There is video on how to repair the worn transfer case
& install a case saver

Need to start by looking at scan tool data on engine first
& it's sensors,then the other modules for trans,etc

Very few transmissions last over ten years,SO !!

2002 duramax ?????? What make , model ???? Chevy - GMC which transfer case , how many buttons on the control switch or is it a dial ? These system's have electronic control modules , can set DTC'S -diagnostic trouble codes . Your best bet , take it to a qualified repair shop.
Transfer Case - NVG 149-NP (One Speed Automatic)
Transfer Case - NVG 236/246-NP8 (Two Speed Automatic)
Transfer Case - NVG 261-NP2 (Two Speed Manual)
Transfer Case - NVG 263-NP1 (Selectable)

More then likely the problem is in the motor of the transfer case controller

Go to Walmart to automotive and they have a tube,it's black on outside,gray in middle,you have to kneed it,mix it up,then apply over hole,it will dry hard as steel,about 4.00.

you would be far better off droping the gearbox into the shop for repair

need to check wheel bearings

if it is an electric shift t-case gm has there own oil not found at auto part stores

I just had my transfer case rebuilt on my 2002 2500 silverado duramax. They told me that the spring that holds the pump inplace in the transfer case wore a hold through the case. He said this was common on these transfer cases. Chevy has not acknowledge the problem. He designed a steel plate, with flanges that holds the pump in place and will not wear a hole in the case again. And as far as the post to engage it once a month, the chain turns even when in 2WHL drive.