Jumpstart the car again and get freebie battery test at wally's world, this will tell you status of alternator output too. Then pull the regulator (2 phillips screws on alternator back) and inspect the teeth for excessive wear. Often $20 reg needs replacement instead of $150 alt. And if you decide on new battery too then grab the largest battery on the shelf, diesels require helluva lot more juice to kick over than gasoline benz you're used to.