eric, i am no expert, but i would take shifer off and see if the bushing on the shifting rod that the shifter fork sits on in trans isn't broke or missing. also check and see if the shifter itself isn't broken. last of all maybe a clutch adjustmet. like i said i am no expert, but some things i would check.
 

I was going to say, I think it may be the shifter un-threading from it's hole!

I had a similar problem a few years back where the shifter came out completely and I had to fix it in the dark in a parking lot. Lucky for me I hadn't changed out my carpet yet and just ripped the carpet back to get access to the boot hold-down screws.

Be sure the tangs on the shifter's plastic nut "wheel" are bent down. They will lock against two raised castings on the top of the tail-shaft and keep it from un-threading itself in the future.

As a side note, removing the shifter several times and bending the tangs repeatedly will wear them out and they will break off. I had that happen this last winter. Lucky for me I had a spare shifter in storage and just swapped it out.