[sethv]

Check the frame near the steering box to see if it's weak or cracked or anything, that's a common problem for the 4x4's. It also wouldnt hurt to check the transfer case fluid.
As for finding out if the motor is original, check the VIN and run it through a decoder, or as rusty said post it here except for the last 6 digits and we'll be able to tell you.

Post pics, we love looking at these trucks!