2004 yukon xl tranny slipping

[neo297]

Hi I'm new to this board. I have an '04 yukon xl 5.3 all time 4 wheel drive no 4 wheel steer. it has 149,000 miles on it and has run great since i bought it about 3 years ago with around 100,000 miles on the odometer. Changed the tranny fluid and filter about 4 months ago. Had no problems then just unsure when it was done last. Suddenly and I mean without any warning or prior slipping 3rd and 4th gear are slipping like crazy. 1,2 and reverse work great but as soon as tranny shifts to third tach races to 4,000 rpm and you can barely keep 55 mph. My question is does a clutch pack go out like that or is the suddenness more indicative of a solenoid or electric sensor issue. I do my own wrenching on my motorcycles and my alfa spider but they are manual trannys not automatics. Also I have had no dash board warning lights I have no obd tester to pull codes myself so was hoping for some info to arm myself with when checking with the dealer or local tranny shops. Any help would be greatly appreciated.
Paul