It is not absolutely mandatory, but it will make it a lot easier to handle the transmission itself. Because of the bulk and balance, it would be wiser to remove the t case first. I am enclosing removal instructions for the transmission from the Tahoe service manual, the instructions for the 4L60E and the 4L80E are the same. If you have a question on any particular part of it, drop me a note. Thanks
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Tighten Tighten the studs and the bolt to 50 N·m (37 lb ft).
Tighten Tighten the stud and the bolt to 50 N·m (37 lb ft).
Tighten Tighten the bolt to 17 N·m (13 lb ft).
Tighten Tighten the bolts and nut to 25 N·m (18 lb ft).