Transmission problems. Is it worth the fix?

Okay so I have a 1997 Ford Expedition and today it started to not shift gears like I will be going 30 mph and I would literately step on the gas and it will still be going the same speed and the rpm rose and it reved and everything. I hoped it was something like my o2 censors or the spark plugs but I got my personal mechanic to check it out and the sad truth is that the transmission is totally messed up. Now I'm at the point where I'm asking myself "what do I do?" I've crashed this truck before and I've probably already put in about $1500+. I'm thinking about just junking it and getting about $500. Or I could fix it which is to get another transmission from a junkyard and the lowest it would be just for the part itself would be $300 plus the labor I will have to pay for by my mechanic which is another $300. Another option is that I sell it to a mechanic who knows how to fix it. I would sell it for $1000 and he would have to fix it. Basically he would be paying $1500 for an SUV. I'm just so unsure at this point. I have no money so if I do sell it then whatever I get is what I have to work with and I would probably have to wait a few checks until I had enough for a down payment or even a used car.

I always ask the same question: Can you buy a usable car for the same or less than it would cost to fix?

If you can, buy it. If you can't, you're just trading one known headache for another unknown headache. If you fix the tranny, then you know the tranny isn't the next thing to go on you.

I would dump it. A trans will run you somewhere around 1700 bucks or more, and that's too much to put into a '97 unless it's in pristine showroom condition.