by the sounds of it there is something going on with the clutch.
first go underneath and with the vehical off, have somone push the clutch in. you will see a cable that connects to the bellhousing, ie the area where the transmission connects to the engine. while they are pumping it you will see at the joining the movement. also look for anything unusual, ie loose cables, oil leaks etc.
second. if everything looks "normal", then unfortunatly you will have to drop tranny.
the problem can be as simple as the "throw out bearing" or clutch/clutch assembly, to something internal in the tranny.
If the clutch hasnt been replaced for a while this is a good oppurtunity to do it anyways.
either way the way a stick"manual" tranny works is like this.
while the clutch is not pressed, ie "out" the clutch plate is in contact with the engine. This is why when you turn it over without pushing the clutch in it lurches.
When you push the clutch in, it disingauges the plate from the enginer via a spring and bearing.
so in a nuttshell it sound as if your clutch isnt "engauging", ie the spring, or bearing are trashed(could be other things as well).
While the clutch is spinning ie in contact with the engine it spins the internal gears of the tranny. Know there could be an issue in here, but the only real way to "troubleshoot" the tranny is to have it dropped, and manualy turn it.(or actually take apart tranny, but this is a hasle, plus you would ahve to replace gaskets).
I'm not sure how difficult dropping the tranny is on a pathy(last time i did it, i was rebuilding the engine, so pulled it through the engine compartment). but from other experiences dropping tranny's you need a couple special tools, first is jackstands, and good jacks, socket set, with alot of extensions(top tranny bolts)), a transmission jack. There may be some misc crossmembers etc. it sound horrible, but mostly if you have the correct tools, it's not the difficult. if this is daunting i would take it to a mechanic.