76 240D transmission problems

My 76 240D transmission shifts into gear but won't move. Engine runs fine. Is this something I could troubleshoot myself? (vacumn leak etc.) Or is it a trip to the transmission shop (or the parts yard)?
Any advice?
thanks,
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How do you know it shifts into gear?
If the transmission IS in gear the car should move, if not, the transmission is not in gear!
I suspect the transmission is dry or its filter completely clogged or the pump is defective. Vacuum isn't an issue when the box is as dead as this one.
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