rough idle on 1998 Chevrolet Tahoe

Example: Sitting at stop lite.Idle starts to run rough in drive position,but when gear is moved to either neutral or park, runs smooth.

sounds like a plug or coil failure. you need a shop to verify and give you a bid.

What do you mean by a coil failure?
ignition coil. there is one for each cylinder and it could be failing by not firing the spark plug.