2003 Disco 2 transmission

It has been driving great, not issues with shifting or going into gear(positive, normal shifting in all the gears from 1st to 4th). While driving normal I stopped at a traffic light and when I took off it shifted from 1st to 2nd and inmediately jump out of gear and went into neutral. I have checked the fluid level (engine running took off the filler plug and fluid came out), checked all the connections for corrosion and found none. The gears in the dash board appears as they should so that rule out XYZ ( I think), my transfer case shift good from low to high and vice versa. Parking gear is good, not flashing M&S lights in the dash, reverse engages and the truck moves well in reverse but when I go into D,2,3 nothing happens and I mean nothing. I probably need to change the transmission (no problem) but I would hate to swap it and after all that work the problem stills. Can someone tell me if it sounds like I need replace the slush box or perhaps anyone can shed a light into this issue..... HELP!!!!!..
ps/ 3 amigos been on related to a bad sensor.

that is strange... i dont think i've ever heard of anyone having a similar issue to that. the transmissions are the one thing on that truck that is pretty solid...