I had a 71 sport suburban for a while in the 90s great car. It sounds like the motor in the tailgate is seized, or it has to be cleaned up. The grease in the gear on the motor gets very hard and causes the motor to work very slowly or not at all. The check is to take the motor out of the door, open the top of the motor, clean out the old grease and put some new grease in the motor. I had to do this on my 71 sport and the glass went up and down smoothly and at a good rate. Check the housing on the motor that holds the window gear they have a tendency to crack, you might have to weld the gear housing if its cracked. I had a 71 town and country that had teh same problem, i replaced the motor with an NOS motor and its worked great for years. If i remember correctly you have to reach in to access the bolts that hold the motor in place, you have to support the glass while you do this. The motor is connected to the inner brace in the tailgate. a 1/4 in drive with short sockets would work in there. The last time i worked on one of these was about 15 years ago, i might miss a step or two.
The 69-71 wagon tailgate only open when the window is down, the 72-73 tailgate can open with the glass up.
Good luck, post some Pics