I have an M1031 with single rear wheels. This is what I found on wikipedia:
The M1031 was the chassis cab. These latter trucks were all rated for heavier 3,600 lb (1,600 kg) or 3,900 lb (1,800 kg) loads, compared to the M1008s 2,900 lb (1,300 kg) load capacity. The M1028A2 (converted M1031 or m1028a1) and m1028A3 (converted M1028 ) models had dual rear wheels.
[12] Many M1028s were upgraded at the company level to M1028A2 and A3 specs—the dual wheel rear end arrangement was a result of the M1028 flipping on its side because of the high center of gravity when carrying equipment shelters.
[13]