The 1991 Nissan D21 Hardbody pickup truck (the 4x4 model you're referring to) doesn't have a transmission dipstick for the manual transmission. You check the transmission fluid level by removing a plug on the side of the transmission case. Consult your owner's manual for the exact location of this plug and the correct procedure.
If it's an automatic transmission, it *might* have a dipstick, but its location varies slightly depending on the specific automatic transmission used in that year and model. Again, refer to your owner's manual for the precise location. It's typically near the back of the engine, somewhat hidden.
