Just guessing but I would tend to agree with SteveH. If it is moving ok when in gear and the clutch is engaged, that sort of rules out the gears being goofed up. Having it stop suddenly when you push the clutch in (and effectively remove power) indicates the emergency brake or the regular brakes are bound up. I suppose something inside the case could be binding up too but I would check the easy stuff first.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang