Data plates with blank dates of manufacture are pretty common. Nobody in service cares about the date of manufacture, only the serial number and registration number matter, so when plates get replaced they don't bother stamping that. You have found the original auction showing no year was listed, so it sounds like the original buyer and the inspector for the state of Texas who approved the bonded title paperwork it made a best guess as to the year.
Since it is a bonded title the state was legally satisfied of the year. There is a good chance this truck went through rebuild, possibly at AM General even, and had some 98 dated stuff that may have let them to that conclusion.
At this point I would not try and change the title date. If what was presented to the state was enough for them to call it a 98, let it be a 98 on paper. But I would represent to any buyer the facts as you know them, that this truck appears to be an earlier one based on the serial number and features but the title says 98.
You are being honest with the buyer, and not poking the government with a stick trying to correct paperwork that really won't change anything if you get.
Your situation is not uncommon. Ask anyone who messes with old Jeeps and you will find that a large percentage of Jeeps are running around with the year of manufacture on the paperwork anywhere from 1 to 20 years off. Like M998's the difference between years were slight on many and most people don't know and it was very common for people to just have paperwork for a Jeep, wreck it, buy another one and just swap plates or for them to do a bonded title and, like was done in your case, make a best guess as to the year. The last GPW I bought was a 1943 based on the frame serial number but came with a title listing it as a 1959 with that serial number.
