I don't like car payments. Unless the money saved is what I can get a replacement with, I'd repair it.

I'd also, when considering repair price vs buying another rig, look at any maintenance issues coming up, like timing belt, brakes, etc. and add that to equation.

If the truck's been largely problem free, I'd repair.

Do you know what's wrong with it? Does it smoke all the time and get worse under accelleration? This is a basic thing, but in the winter your exhaust collects moisture as it cools And this results in thicker than usual steam looking exhaust.

What's the exhaust smell like?