the rear axle with disc brakes just went out and the tow company gave a $1700 estimate. Could we put a used axle in for cheaper.

You'll have to call local salvage yard for price.. Gear ratio will have to match, salvage yard should have info... Has a 'mechanic' confirmed differential is SHOT?.. Was that estimate for repair of existing rear axle?.. Do you know what happened to it, ie..ran without lube, bearings or gear problems?