The odds are that one of the segments of the speedometer cable is damaged. All data should show where it runs. Start with pulling off the lower segment which runs into the transmission and (while holding one end to prevent movement try to turn the other end a couple rotations... if it spins relatively freely that segment should be replaced, if not, reconnect it to the other half, turn and have someone check if the end in the cabin turns freely (with the cluster removed, of course)... if the cable is good.
Then try driving the speedo with a drill (pressing in a soft dowel about 1/8" should fit the speedo's interface.
The last part is the speedo drive gear in the transmission which the cable connects to, but save that for last as you'll have to drain the gear oil to check it (you can check to see if it spins freely while it's in though; If it does, it's shot).
NOTE: the only part which should move freely is the speedometer itself (naturally, the cable will if the other end is unbound though).
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