I could hear a very loud multiple thud sound under the car on the first gear.

What is your question?

Multiple thud sound on the first gear only specially when the road is going up. The thud could only heard on the first gear and after that no more. My mechanic said it might be the transfer case, He is charging me too much for this that i can't afford to spend for this old car, please kindly help or any suggestion. Can you tell me if the mech.is right, Thanks in advanced.