2002 / 44,000 mile Transmission Failure

I have a new problem with my 2002 GMC 6.5L Turbo Diesel 3500 Cut-a-way van. (3500 Series Van with Stahl Utility Body Dually)

Yesterday van shifts like someone installed a shift kit in it...bangs gears and locks up early on OD.

Today... I come off the highway, stop at the end of ramp, wait for cars to go by, try to take off and feels like its in neutral....but its not.

Some fine high pitched noises and she reluctantly goes untill the next stop sign...same thing again.

First thoughts are low tranny fluid....but when I pulled the dipstick, it had plenty of trans fluid....But!! ..it was grey and smelled burnt.

This van is maintained religiously every 5000 miles, it has never towed anything (has no trailer hitch) never plowed..and carried about 8000 lbs of weight all the time.

Any idea's?

Thanks
JLC