1/2 Ton or 3/4 Ton?

[dunkinhard02]

I just bought a 68 Chevy pickup that originally left the factory as a 6cyl 2wd model. However, a previous owner has converted it to a 350CI, 400 Turbo Tranny, and 4wd. It has 6 lug axles, and discs up front. The rearend is a 12 bolt. I was told that the axles and frame MIGHT be 3/4Ton, but the previous owner wasn't too sure. Is there anyway to distinguish between years on these axles/frames, and how can I tell if the frame/axles are 1/2 ton or 3/4 ton? The front manual lockers on the front axle are inoperable, and I need to know what year/axle to order them for (they attatch to the axle with 6 small bolts with allen heads). Thank you in advance.