Hi rbarr,
I have made a number of changes to your model, but I think it now does what you intended it to do. I have moved the pivot point nearer to the front of the desk, increased the height of the desk top and chamfered the front of the top, all to provide clearance to allow the top to raise to a reasonable angle. I have also added an extra pivot and hinge at the back with a second slider. The back slider was trying to keep the top at right angles to itself, while the front slider was trying to tilt the top and this is part of what was causing the problems. I also re-grouped some of the joints. The original hinge joint was not grouped with anything, so that is why it would not move with the rest of the mechanism. Hope you like the result and let me know if you need any more help.
Kev.