As the title states, I have a vibration at idle (600 rpm) while in gear. It does not vibrate in Park or Neutral. It also does not vibrate once I let off the brake. I just bought the truck.

Specs
2004, Chevy Silverado 1500, LS, LM7 5.3, 4L60e, 3.42 rear gear
98,200 miles

Thanks for any help in advance.

 

do the RPM's stay steady when its doing it?
I'd check the mounts, engine and trans.
have it in park, put the e brake on, start the vehicle and have someone else hold the brakes and go from reverse to drive and back to reverse a few times, watch and see if its rocking a lot

 

Yes the RPMs remain steady. I don't think it's an engine related issue. My thoughts are either the mechanical fan, or as you stated the engine/transmission mounts.

Also, I should have said it in the first post, but it is 2wd. Also, when I come to a stop there is a "thud". Makes me think even more that is is a mount issue.

 

I know this is a long shot. My truck does the exact same thing. My vibration only starts once the truck is warm. I also notice that it is worse if I am stopped on a hill. Did you ever find out what the problem was? My truck is a stock 4.8L 2wd automatic crew cab with 88k. My vibrations started at about 80k.