Like Kennedy suggested, look for a leak at the top of the engine, and work your way down. Eliminate the easy and inexpensive possibilities before suspecting something more.

That said, these engines employ an aluminum bellhousing adapter that bolts onto the rear of the engine. Aside from allowing the trans to bolt to the engine, it also has coolant passages that distribute coolant to each side of the block. I know of at least one that developed a coolant leak. You may have to pull the engine to re-seal it - or it might be done by first pulling the transmission, and working beneath the truck.

Jim