I've got the M5OD in my 1994. This has only happened a couple times this month but when I put it in reverse the truck lurches like its trying to take off. It felt like I popped the clutch even though the pedal was on the floor. The parking brake was still set so at least it didn't go anywhere. It was hard to get in gear and kind of popped in when it did engage. The engine didn't stall though.
My first thought is to bleed the clutch. Maybe the fluid is going bad and needs to be changed or somehow air got into the system (which would be another issue in itself). Anyone got a different idea before I go into that?

 

Sounds like the clutch is still partially engaged. Check for leaks in the hydraulic system.