02 gmc Sonoma has to be rev up over 2500 rpms before it moves! 4l60e automatic

My 02 gmc Sonoma started having trouble moving when its first started & has to have about 2500 epms to get it moving forward or reverse, once it starts driving, it drives pretty good,but the rpms are hi when it shifts. I had it at a shop & they checked it with a scanner & said it’s probably the clutch pack & I mite get lucky with a fluid & filter change or it mite not drive at all,if I do a filter change. They didn’t check any solenoids for ohms etc & I took my truck to this same shop in October & they didn’t figure out the hi rpms,but now they said if I replace the transmission the hi rpms will go away, so I don’t know what to do for sure now.. id like to check the solenoids for ohms by back probing through the computer harness but can’t find a wiring diagram. All of this started with a p0122 tps code & now since it got cold the shift problem began. It’s a 4l60e automatic