99 yukon denali my transmission doesnt go into overdrive

Boy, do I feel for you...I hate to say it, but as a GM mechanic for thirty-five years, I can promise you that this 4L60E transmission is dead. Unfortunately, 4L60E's are relatively weak in stock configuration. I have repaired this exact problem many times. Here's what you do:
When you get the transmission rebuilt, tell the builder that you want, as a minimum over a stock rebuild, the following parts installled...you want a hardened input shaft, The Beast sun gear reaction shell, the 29-element Borg-Warner input sprag, the Alto master rebuild kit with all molded rubber pistons, all Alto Red Eagle frictions and full-thickness Kolene steels throughout (including in the low/reverse clutch pack--reverse is a weak spot for 4L60E's), 13-vane pump kit, the Sonnax .490" boost valve, a new reverse drum and an extra-wide (2 5/8") carbon fiber power band, a Sonnax cast-aluminum pinless forward accumulator piston, the Sonnax heavy-duty 2-3 shift valve, Sonnax "Super Hold" 1-2 and 3-4 servos, the Transgo valve body separator plate, and a 15,000lb-capacity auxilliary transmission fluid cooler, installed in series in addition to the transmission fluid cooler core built in to the radiator. This build will cost you about 600-800 bucks over a stock rebuild, but your transmission will shift firm, crisp, and reliably every time. You will think you have added about 100 horsepower to your engine without having even touched it. This transmission will tow anything you hook to it, will yank any Ford, Toyota Tundra or Nissan Titan all over the parking lot and burn up their transmission, and it will still be going strong when you sell the truck someday. GM trucks are awesome with the proper build on the transmission. I have three built this way, and tow a thirty-three foot camper and a 10,000lb car-hauler with all of them.