Diesel Bombers - 94 gmc 3500 dually. 50 mph to nothing

This is my first post. I did some searching prior to posting about this topic but was unable to find anyting pertaining to my specific problem.

Today,
Girlfriend leaving work in her 1994 GMC k3500 dually 4x4.
Going 50mph, lost drivetrain. She heard nothing, smelled nothing, nothing leaking just bam, stepped on the gas and the truck would not accelerate.

She pulls over, tries to put the truck into park, but it just grinds. So she shuts it down, puts it in park. Re-starts the truck, puts it in drive, nothing, so tries to put it back into park...again it grinds...she has to shut the truck down completly before putting into park or major grinding ensues.

WTF happen? how can you be going 50mph, the truck running great, to no transmission?
Im currently at work, and havent had a chance to look or play with it at all, but my experiance with transmissions / transfer cases is limited.

I figured I would speak with some experianced minds prior to starting the troubleshooting process....

I recently did a lot of front end / brake work to this truck to get it back on the road. Its been on the road for the better of a month and has been running / driving great. I personally KNOW all fluids are full, oil changed, ect.

So what failed on this p.o.s now?

-Duallydude:argh: