Hello and welcome to JustAnswer!
A pinion gear is the gear in the differential that is basically mounted to the driveshaft. It engages with the ring gear in the differential, and that is what transmits power to the wheels. The pinion gear comes though the housing of the differential, and the pinion yoke mounts to it, the yoke is what bolts to the driveshaft. Where the gear comes through the case there is a seal to keep the differential fluid in. It's not uncommon for these to leak on the Dana axles in the Jeeps. The front pinion seal is the pinion seal on the front differential in your Jeep.
I fully understand being short on money! It's hard to say how long you could go without fixing it. It depends on how bad it's leaking. If it's just seeping a little, you're ok. If it's leaking badly you could run out of fluid, resulting in complete failure of the differential, that would be well over $1000 to repair. Take a look at it and see how wet it is. If you see actual drips it would be more urgent. I would recommend keeping an eye on the fluid level in the meantime to make sure it doesn't go low.
The price of the seal looks to be about $15 and it would be about 1 hour of labor to replace it. You may also have to pay for a quart of fluid to top it back off after the seal is replaced, which wouldn't be much.