My '92 service manual says the same as what Grey Wolf posted.
3. 54 = 12,000
3. 90 = 13,000
4. 10 = 13,500
4. 56 = 14,000
My take on this is that the 5. 9L engine that it refers to is the gasser and not the diesel.
The 92 owner's manual which specifically covers the diesel shows the following GCWR (total truck,trailer,cargo,people & fluids)
D&W 250/350 - - - - - - 3. 54 - - - - 4. 10
Man 5-speed - - - - - 16,000 - - 17,000
Auto 4-speed O/D - -12,000 - - 12,000
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Al