First check and make sure there are no leaks. Follow the fuel lines back down from the engine and check for drips. Also check around the lower intake manifold /fuel rail.
Now it is very common for the OHV 4.0L to smell like fuel, this is because the canister purge valve has failed. When they go bad (lots of miles) they lock in the open position, meaning the charcoal canister is being "purged" the entire time the truck is running, causing the fuel return line to always be sending fumes/vapors back to the tank, eventually pressure builds up and the fumes start escaping up under the hood. Normally the computer tells the canister purge valve to open and close.
Canister purge vavles are about $20 from Napa, takes about 30 minutes to replace, but be careful the plastic fittings break easy!!!
On a 91 the canister purge solenoid should be up near the drivers side headlight, hanging right near the charcoal canister. It is a little larger then a film canister and is brass in color. You may find that boiling the nylon fuel lines in hot water can make the new valve much easier to install (press fit)
Good luck!
