Tire question: 81 J10 Honcho

[Jeff in north Texas]

Today I picked up my 81 J10 and drove it home, about 25 miles or so. The steering is tight, but the truck really wandered side to side on uneven asphalt road surfaces.
I think it is the tires. They are a very agressive tread, worn unevenly, and are big: 31x10.5 R15.
I want to replace them with tires about the same size that came on the truck when it was new.
What size tires would probably have been on this 81, J10, Honcho truck. A sticker on the door, which is very hard to read, looks like it shows a tire size of "10x15".
Thanks,
Jeff

[Tornado230]

That was the standard tire on the J-truck with the 8" rims.
Basically a non-radial 31" tire.