Was the Diff Noisy, if not then I wouldn't worry. The Diffs are pretty long lasting and tend to get pretty noisy long before they ever give up the ghost. Be aware it is quite possible it has never been drained before so some build up of metal swarf is quite normal. My 80 had a noisy diff when I first got it and when I took it out the crown wheel and pinion were well worn. Changed the diff for another one which was still a bit noisy but in better condition, drove it for best part of 100K miles, drained it once and put wynns in it and it didn't get any worse in all that time. Only time I'd be worried about the front diff being a problem is if it is well worn and then you go and stick longfield axles and birfs in and put 37 inch plus tyres on it, it will probably let go then.