So we had a spot of nasty weather last week (rainy slippery roads) and I shifted my 2000 2nd Gen 1500 into 4H. I drove it those two days, 20 miles total for both days and promptly forgot I had it in 4H and then drove it an additional 140 miles to the beach and back. The speed on that trip varied but got as high as 70 mph for lengths of time as long as 40 minutes.

Now I'm hearing a rattling sound on the front end when I'm in 4WD. In 2H there's no noise. Ideas, other than I may or may not be stupid?

 

Should be fine then.

 

Have a look at the front U-joints.

 

Did the 4WD light come on? Hard not to notice that.

 

I'd check the u-joints on your front axles and the front driveshaft. I'd lean towards the axles more as they don't like to turn under stress on the road. In dirt or snow, there is some slippage.