I think JD offered the 8640 from 78-79 thru 82-83. If it is a 82-83 model it is 35-36 years old. The owner has probably had it looked at a JD Dealer and was floored by the repair estimate. So he started looking for a independent to take on the job. If I was Mobile Mechanic I would pass on attempting repairs. That is one huge tractor.
Most JD Dealers will not discount parts to a independent. So Mobile Mechanic would have some expensive parts to eat if his guess as to what the problem is, is wrong and he has to move on to more expensive parts.