Just a process of elimination. Take the belt off the front. Still vibrates? Yes=not there. Put it back on & unbolt the torque converter and slide it back. Still vibrates=not the converter or trams. While the converter is back take a look at the flex plate for a missing weight.
Is this vehicle a virgin? Or has some dyi guy been screwing with it? If the latter it could even have the wrong flex plate.

 

Hey Yeah, i never thought about sliding the TC back.. I really didn't know you could do that without breaking the bell housing loose and pulling the whole trans.

The belt up front made no difference though.

 

Hi
I know my post will be something quiet strange as its a an old post,after reading alot of signs and symptoms and issues with a simmilar car which has the 5700 vortec engine as mine does which has a rough engine vibration at idle and mostly noticed at 2000rpm then goes normal and power is abit lacked on the engine.
i decided to share my experience with the solution which i came of to repair my car which had this issue for a while and non of the techs could support me neither did GMC .I am from Oman and its abit dificult to find junk yards for second hand spares ,and rarely can find and experienced person on these cars.
i would like to appologise for my english as im not a native speaker but it should be delivering the right thing to do.i love GM cars in general and im happy to own alot of th
First ill start with my cars:
1-GMC YUKON GT 99
2-GMC YUKON 98
3-GMC YUKON XL 01
4-GMC YUKON DENALI 2012
5-GMC YUKON GT 97 (Modified and engine swaped)

My issues were with 98 and the 99 yukons
which had almost the same issues such as the engine vibration,security ,trans ,power steering , Fuel gauge shaking,airbag light and more.
our topic is engine vibration so ill discuss it here and other issues ill post them on other forms.
sorry for the long intro and now i start:
My car has done 120k miles then started to have this funny thing which was amystry to solve as anyone i ask he says something diffrent than the other,some said its engine mounts and trans mounts ,some said fuel pump and some said the injectors so it was diffrent opinions which made me check alone then repair it .
I decided to replace the spark plugs with anew set of wires,at the start.so i went ahead and removed the wires and checking them one by one till i found a broken wire wich had to release the spark and hit somewhere on the engine block which caused a rough idle due to lose of spark power and i figured out then why that happened,obviosly because of the manifold heat which touched the wire when it moved down to make it melt,then i replaced the spark plugs with new ones,and bought new sets of Wires,cleaned the throttle body,cleaned the injectors with cleaner in tank and started the car again,,and its was like a game to me as the tricky part come when you want to remove the plugs and the wires from the last parts of the engines so all you need is spark plugs and wires and making sure to hook it to the place which is supposed to hold on and confirming its not touching and metal ground piece and that worked for me i hope this will work for everyone has the same issue.
thanks for all for reading i wanted to share this experience which may help people rathar than spending alot of money to repair a silly thing.
good luck to all