It might still be “Early Days” yet about having to Pull the Engine. Most of the time, when the Flex Plate Cracks begin to propagate… they center around the TTY Flex Plate Bolt Pattern and eventually the center, Thin Mild Steel area separates from the outer plate that is held in place by the Three Torque Converter Bolts… almost like a “Cookie Cutter”.
This means that without having the Torque Converter completely out of the way… the damage there will be difficult to either confirm or eliminate.
Pulling an Atlas Vortec Motor out of any GMT360 Body and Frame
is No Trivial Matter. So before then... Your Next Steps in this Diagnosis should be to view this Linked Video and follow the Basic Suggestions made by the VOP (Video Original Poster) herein to seek and find and CONFIRM the presence of any Bad Connecting Rod to Crankshaft Journal Bearings.
Thankfully, because the GM 4.2L Engine is an In-Line Design with complete and Open Access to the Spark Plug Ports… performing these procedures is relatively easy:
This Test is Definitive for Bad or Failed Connecting Rod Bearing(s) and so the necessary Engine work
(...and that means doing EVERYTHING NEEDED to it while the Motor is OUT) on the Atlas Engine is ever so much MUCH EZR to perform with the Motor Mounted on the Engine Stand than when trying to work while Laying on your Back under the SUV.
This repair HAS recently been quite successfully achieved by
@gmcman, so his counsel to you will have Great Value. However… if you do NOT find this to be the Con-Rod Failure to be the Case… I’m still suggesting that the Flex-Plate will be yet another culprit to be eliminated.
Mind you, it will be almost as difficult... but not as much work as yanking out that Engine.
Of course... if you can indulge your Shop for the Lion's Share of this Labor... making the immediate decision to just R&R&R (Remove, REPAIR, Re-Install) that Motor will come with a lot less agony.
“Good Science involves investigating (or changing) only ONE Variable at a Time...”