The NorthStar utilizes the 4T80E transaxle. The 4.9L engines utilized the 4T60E transaxle. The two transaxles are similar but not at all identical. Solenoid kits are not the same or in the same location. Typically the 4T60E transaxle does not require the upgraded solenoid package.
Slip in forward gear engagement suggests that there is a lockup problem. I would normally start with a transaxle fluid change (NOT FLUSH), along with an internal inspection/cleaning. The 4T60E transaxles should throw a code when they begin to slip.