did you verify that the end that goes into the speedo isnt spinning?
try spinning the end that goes in the speedo with your fingers, if you can, it isnt seated into the gear in the t-case. if you cant spin it, then that end is ok. and its the dash side that is giving you the problem.
plug it back into the dash speedo and take it off the t-case. chuck the end into your cordless drill, and you have to turn it counter-clockwise or in reverse in the drill to check the speedo head.