I can't figure this out....
 

I picked up my truck from the tranny shop today.....like before it went into the shop it runs rough when it's in gear but when it's at idle, it runs perfectly normal.....someone told me that this is a sign that the engine is dying and I'm colse to needing a rebuild....is this true?.....it's really bad when I'm slowing down to a light and then I stop and then it starts to shake....but if I put it in park, it runs like nothing happend.....any help or info would be apreciated.:gmc:

Could be several things, or a combination.
Timing. Vacuum advance, or mechanical advance, or sloppy distributor.
Worn out timing chain.
Vacuum leak, intake to head, or carb.
Leaky carb at throttle shaft.
Bad spark plugs or wires.
Bad points or dist cap.
I'm sure there are ohter possibilities...
I'd start with a full tune-up and see what you find while you're doing it.

Or the idle is just set too low.

What do you mean by "runs rough"?

it almost feels as if the timing is off but I know it's not the timing.....because it runs so smooth at idle....but does this change when it's in gear?....also...I just had the rear main seale replaced....do I need to adjust something after this has been done?

I am haveing the same problem on my Fresh 383. I think I have it narrowed down to a vac leak, but I don't know where. I got the dist. from my brothers truck we tore down a few weeks ago, and it did the same thing. I tried changing the spring and weights, since it seemed to be a timing thing at first, but that isn't it. Now I have to find the vac leak. It's bu!!sh!t about this meaning it needs a rebuild. When I find out for sure about mine I'll let you know. If you find out about yours first shoot me a line.

Give us some history on this engine/truck, we need more info like is it stock, modified and what has been done to the engine recently?

it's a 350 and everything is stock except for the Carter carb and K&N air filter.....I might be on to something though...the exaust manifolds are completely shot and there are a few exaust leaks now......also about a year ago before it sat for a year.....the water pump went out and the engine overheated to the point that it shut itself down and then when I lifted the hood, steam was coming out around the edges of the intake.....so I'm wondering if this too had something to do with it....but that's the history of bad in the last 6 years I've had it......Oh yeah....I burned out about 3 gallons of desile fuel 5 years ago......didn't do any harm though....just alot of smoke.....I'll tell ya guys.....I'm closer and closer to putting in that rebuilt Olds 350 longblock....but I' don't know what I need in terms of mounts for engine and pumps and alternator.

Timing and idle were set with the truck in gear, right (hope I remember that correctly)
Check the intake and carb bolts (nuts)

If the truck has power brakes then with the vehicle running pull the hose loose from the power brake booster. If engine doesn't kill then your booster is leaking vacuum. I know this sounds weird but the vacuum as explained to me is also engine vacuum reservoir. Been through that before.