Under the cowl plate is the unique serial portion of the VIN number. Pull the wipers, 5 or six screws I think, and it will be on the driver's side.
Differences between 76 and 77 are pretty much all down to the bed which being a stepside erases completely. The 77 bed Amber turn lenses, less tie down loops, but again that is the fleetside bed so it doesn't make any difference for you. I also looked at the wiring diagrams and I did not see anything even remotely significant to look for. I think if the police can find a correct vin on it somewhere that is about the only way they are going to really want to let it go. If you haven't already you should put together a list of every single thing you can remember about it that is distinctive and the more hidden the better. I know some intimate details on most of my rigs, spliced wires in the harness and that sort of thing. The more of a case you can make for it being yours the better.
Oh yeah, don't forget the door sticker. If they painted over it that will also be a place for the VIN to still exist if you can get the paint off cleanly. My other suggestion would be to track down the other truck. If you can prove the other VIN still exists on the other truck that would help too. Last registered owner might know something, but they might also be in on it too. I'm not sure you can get that info, but the cops should be able to figure that out if they aren't sitting on their thumbs.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
