99 SVT with 5 speed. All of the sudden, no gears. Clutch works fine, shifter goes into all gears. Constant grinding noise in all gears with clutch released. Speedo does move as engine rpms increase. Thinking it's the diefferential gears? Any ideas?

My first guess is a blown diff. What is the actual scenario in which this started happening?

Actually it was not being driven hard at all, just pulling out of school parking lot in straight line. No indication that anything was wearing out. Could it be the spider gears? neither wheel turns, it does move a little in reverse but under slight load, loses all traction again

The spider gears only work when one wheel is turning faster than another, like going around a corner. They are not used when going in a straight line either forward or reverse. I think you have a CV problem. The speedometer only works when the differential gears are turning. Like you say. It could be the differential. Go to a transmission shop for a consult. They wouldn't charge normally for an estimate.

Actually post this on fordcontour.org. Terry Haines, the head drivetrain engineer of the MTX-75 transmission, is a member over there. Nobody knows this trans better than he does. He probably could tell you just by the symptoms your having.

Turned out to be the differential bevel gears (spider gears and drive gears) were wore down to nothing. I found a replacement differential and reinstalled and all is good