Since it's started getting hot outside I have noticed a distinct and strong gas smell in the cabin of my 92 4.0
It's almost unbearable, my GF can't even stand to ride in it.
What could be causing this?

 

The 2 straps that hold the gas tank up tend to rub through and cause leaks. This is most likely your issue. You really just need to poke your head under there and take a look for leaks. Follow the lines from the tank to the filter up all the way to the fuel rail. You'll find the leak.

I'd be willing to bet the tank is leaking due to the straps rubbing on it though. Just replaced the tank in my dad's 95 because the straps wore through it. Be prepared to break and buy two 30 dollar j hooks from the stealership! (or make some like I did)

 

You'll need to invest at least a few minutes under the body & hood.

"My Cherokee makes a loud clunking noise in the cabin."

Sorry, I couldn't help myself.

 

I opened up the hood and discovered gas leaking from what I can only imagine is a fuel injector (the front one)
How hard is this to fix? Just need to replace the o-ring right?

 

Yes you need to remove the fuel rail to get to the injectors. If your doing one you might as well do them all in one shot

 

Yeah I just looked at advance, looks like about $10 for 12 o rings. I've looked and can't find instructions, know where I can find a write up?

It looks like it's leaking from the bottom and around where the plastic clip meets the metal.

 

Use some lube when pressing them back in. I've ripped a couple on my Cherokee. They seemed to be a tighter fit than usual that or cheaper o-rings than Motorcraft that I'm used to. Dab some dielectric grease or petroleum jelly on them or they'll probably leak again like mine did.