Definitely check the vacuum and also check carefully for cracks in the rubber hoses but most long sitting motors I have played with will move quite well unloaded. As soon as any drag or load is given to the motor by the wiper mechanism it all but stops.
In spite of having adequate vacuum there is still a good possibility the seal around the sides of the paddle which makes contact with the sides of the vacuum chamber has become dried or worn from the constant sliding against the chamber and is not making a tight enough seal to avoid a vacuum leak so motor cannot provide full power. There is also a possibility gaskets have dried and shrunk back enough to provide an internal vacuum leak.
You can try lubing per the instructions on page 52 in the service counselor article entitled
Lubricating Windshield Wiper Motorshttps://packardinfo.com/xoops/html/downloads/SC/SC-VOL23NO10.pdf but I would almost bet the motor will require a rebuild before it works properly. You can send it to Ficken Wiper Service and in a fairly short turnaround time get back a motor that runs as new.
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