Cooling System Trouble

I have a 94 Toyota 4Runner. Recently, I was driving it and all of a sudden coolant started flying out from under the hood onto the windshield. I pulled over and opened the hood, and the cap for the coolant container had popped off. I replaced the cap and let the car sit to cool down. I started on the highway again, just trying to get to eh nearest gas station, and on the exit ramp there was a loud "thud" from under the hood and coolant sprayed all over the windshield again. I pulled into a gas station and popped the hood. This time, the cap for the radiator had ripped off (part of the radiator came with it) as well as the cap for the coolant container (they are connected). I had the car towed. At the mechanic, they replaced the radiator, and then told me that the car was having the same problem. I don't know enough about cars to know if replacing the radiator was the right thing to do at this point. However, the mechanic thinks that the problem is an internal engine problem, and that the car is basically dead. He suggested rebuilding the engine or replacing the engine. Are these the only options? Or do I need to get a second opinion from another mechanic? It just seems illogical to me to rebuild the engine when the problem seems to be in the cooling system? Any ideas/suggestions from anyone who knows a little bit about the 4Runner would be great! Thank you!