Big mechanical problem
 

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 metallic 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. Lots of 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?

sounds like a tranny or t case problem....I would look at the t case first....drain the fluid and see what you see......

Good idea, I'll do that today.

I looked at the tranny fluid. It looks fine, but does not smell normal.

If you had thrown a rod, you would not be able to sit still and rev the engine. I think it's in the transmission. Do you have all your gears?

How are the u-joints?

Yes it has all the gears. But it vibrates a lot at low rpms in each gear and loses power.

I plan on checking the u-joints and the transfer case tomorrow.