That is the same style lock that is on my '77 Excella 500. I remember locking the unit on the way back from a trip, unlocked it when I got home and it would not open. A call to Andy at Inland and I ordered a new tumbler with 2 keys. I remember this being around $54 almost 3 years ago. Instructions call for you to drill out the tumbler, pull out a snap ring if it doesn't fall out and then insert the new tumbler. I haven't had a problem with it since. They sure are right about pushing the door in slightly before pulling the tab. I usually put my thumb on the tumbler part of the lock and pull as I push in. Works for me.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system