Could it be the rear main bearing?

I bought a '90 4x4 Truck w/the 3.slow for my stepson back in 1995. At 58k, noticed a low freq knock at low speed under load. Turned out the factory had installed the incorrect main bearings when the truck was new, and there was excess play at the rear main crankshaft journal. Toyota fixed it under warranty, and replaced the rear main seal, which had started to leak. Drove the truck another 70k with no further problems.