A shoulder bolt is just the portion of the bolt that is not threaded. You see them all the time in long bolts. Only the first inch or so will have threads and the remainder will not. It's this unthreaded portion that you use to make your planets out of.
When you unbolt and remove the motor for testing you will be able to lift the window by hand. You can wedge a piece of wood in there to hole it up or use duct tape on both sides of the window glass up and over the door frame.
If there is a spring it is going to unload when you remove the last bolt holding the motor. Be careful to hold onto the motor as you remove the last bolt. The will probably be 3 bolts holding it.
The idea of the vise grip is to load down the motor. A lot of times the gear will turn with no load even if the planets are broken into a million pieces, but once you put that vise grip on there and hold that gear back it will slip if the planets are shot. This is also a good time for you to learn about a planetary gear system. Once you finish this repair you should be able to explain why a planetary system is superior to a ring and pinion even though they are used to do the same type of job manys times.
I rarely buy replacement motors for Fords and Linclolns on these older vehicles. The shoulder bolt will never fail. The motor will burn up first.