$250 for a used diff isn't too bad. How much for a shop to put it in you reckon?

Something else I forgot to mention. When I pulled the cover off the air filter, I stuck my finger back into the hose that connects to it and there was some (not much really) oil in there. The air filter was clean, but is this normal too?

And, what is that small/short hose that connects to the top of the engine, just behind and to the left of the oil cap? It was a little loose and wet around it.

Forgot to mention...it's an automatic.

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1994 C280 204k miles