Whenever I come to a stop in my S10, I can hear a rattling noise coming from somewhere under the truck; (don't know if it's the engine or what), and also, the truck starts shaking. It doesn't shake and rattle when I put it in park, just when I have it in drive and am stopped on the brakes. (It also doesn't seem to do it as bad on the grass as it does solid pavement). Any ideas what could be the problem?

motor mounts, or transmission mounts.
Also check to see if all the exhaust is tight, although I don't think that this will cause the shaking

Is it a bad thing?

Will I end up breaking down or wind up having to replace something expensive?

If it's 1 or more of those 3 things that I mentioned it'll be pretty cheap to fix properly. Possibly free if it's just some loose bolts in there.

I had the same problem, the exhuast was loose at the manifold, which meant that the o2 sensor wasnt getting a true reading. which in affect caused the noise and shaking. Not saying thats exactly what it is but thats what happened on my '00