1991 k1500 350 automatic 4x4.

Was driving back from the lake yesterday towing the boat. I frequently switch between (D) and D as I'm driving depending on the hills. Was driving fine along a flat stretch, suddenly heard a bang and the truck started vibrating a lot at low rpms.

Got out, looked underneath, didn't see anything out of place. Looked under the hood, didn't notice anything.

Got back in and drove home. Plenty of unnatural vibration at low rpms. Loses power going up hills. Drives okay on flat stretches and at high rpms. When in Park or Neutral the engine sounds fine when it's revved high. But when in gear there's a lot of low-end vibration.

I'm thinking transmission or transfer case blew a gear. Or maybe the motor threw a rod. What do you guys think?

 

Are the U-joints solid?

 

driveshaft maybe?
you said vibrates at low rpm's.
does it vibrate only while truck is moving in drive or while truck is stopped in drive too?

 

Problem solved. Turns out one of the spark plug wires melted against the exhaust manifold and shorted out. This was causing the engine to miss, which was noticeable at low rpms and not noticeable at higher rpms.