Okay, here's the deal:
The front diff housing HAS to rotate for your speedo to show something other than zero. The speedo needle is magnetically coupled to the speedo cable, which is mechanically connected to the front diff with a set of worm gears. The driving worm gear is ON the front diff. So the front diff HAS TO ROTATE!
If your diff is rotating, but your wheels aren't rotating, the rotation is not being transmitted from the diff housing to the axles. This leaves really only a couple possibilities. A)One or both of your axles is not pushed in all the way. or B)Something is really wrong with your spider gears.
In the A scenario if ONE axle is not in all the way, the diff can spin and not impart ANY load on the other axle. The spider gears (in an open diff) will just spin around and do nothing.
I don't think scenario B is plausible without a ton of noise.
However, this doesn't address how the front diff could spin, yet the car doesn't move. Because the Viscous coupler is STILL trying to send power to the rear wheels. (If there was something wrong with the VC or center diff, it's likely the front diff wouldn't spin either.)
Now, if the center diff was fine, but the VC fell off, that might be a possible explanation. Since open diffs send power to the path of least resistance (the slipping wheel) a missing VC would mean that the center diff spider gear that is connected to the t-case output shaft would be effectively anchored. This would send ALL the power to the front diff. However, there would still have to be a disconnected axle in the front diff to get the speedo to still move.
Another possibility is that the output shaft to the t-case was not secured and somehow is ‘falling out’, i.e. no longer connected to its respective center diff spider gear. In this scenario the VC is still functioning correctly. Since the VC would maintain a relatively matching speed between the center diff housing, and the t-case output shaft spider gear, all power would be sent out the opposing spider gear and thus to the front diff. However again, one or both front axles would have to be disconnected. (The other possibility in a similar scenario is that the t-case is not installed all the way, OR the t-case input is stripped, OR the driveshaft is not connected to the t-case, however these would be noisy or messy or very noticeable IMO.)
So I would suggest a couple easy checks:
1)Jack up ONLY the front of the car. Grab one front wheel and spin it. Does the other wheel spin, and spin in the opposite direction.
2)Jack up the front of the car in a way that it only lifts one front tire. Try to spin the tire. Can you spin it?
2)Jack up ONLY the rear of the car. Grab one rear wheel and spin it. Does the other wheel spin, and spin in the opposite direction. (This will big much harder if you have an LSD.)
3)Jack up the rear in a way that it only lifts one rear tire. Try to spin that tire. Can you spin it? If so can you observe the driveshaft turning?
As for the clutch slipping, I think that is absolutely not the problem. For the people that thinks it is the clutch, please explain how the front diff is turning and giving a MPH reading through the speedo, yet the car isn’t moving.