My 2000 RCSB has a strange vibration when in gear. Its only noticeable when Im sitting at a light but its really starting to get annoying. Its done this since I bought it I think. I was rear ended about 3 weeks after I bought it ( I bought it new in July 2000)and it was repaired at the dealership were I bought it. I have taken it back there time after time and they cant find anything wrong with it. If I shift to neutral from drive at a light it goes away. Could My drive shaft be the culprit? I hate chasing down little things like this but its really starting to annoy me. It has a 4.3 and 4l60e if that matters. Thanks in advance!

 

How could it be the driveshaft if this vibration is only when sitting at a light in gear? The driveshaft doesn't spin when you are sitting still.
The reason the vibration goes away when you place it into N is because the load is being taken off the engine.
The vibration is from the engine. It could be just the way the 4.3L engine is going to run. One of our company work trucks has the same engine and tranny as you. It's a 2002 model. It runs rough when sitting still with it in gear. I think you either need to accept it, or step up to the V8 class.
Try running some Seafoam through your brake booster vacuum line. It will clean up your intake, valves, and combustion chamber.
Also clean your MAF meter and throttle body with some throttle body cleaner.
Don't touch or wipe the MAF meter after spraying it. Just let it dry naturally.
These items should fix the idle vibrations.
There is a Recall on the distributor cap as well. The original cap and rotor is prone to corrsion due to the A/C line running over it. The A/C line actually drips condensation onto the distributor cap causing corrosion later down the road. When it corrodes, your truck will run really bad and possibly start missing. This happened to our company truck as well.
GM replaces the cap & rotor and insulates the A/C line running over the distributor cap.
This should be done under warranty from a recall notice. You should ask them if yours is covered under a recall warranty for this repair.
Good Luck.

Jim

 

looks like an engine swap is in my future. I never did like the 4.3 but it was all I could afford at the time. Thanks for the input.

 

Yep I'll second that weird vib in the 4.3. After driving a 4.3L ex cab short bed truck for a week i won't complain about my turn of a 5.3

 

modified exhaust? Possible loose bracket from exhaust.

 

Nope the drivetrain is bone stock. I've put it on a lift and checked it out myself more than once and just couldnt find anything wrong with it.