Sounds like the 3-4 clutch is fried. The most common failure of a stock 4l60e. Often times the input housing is poorly machined from the factory and for whatever reason will cause the 3-4 clutch to burn up. It could also just be from hard driving as well. Like I mentioned, that is the weak point clutch wise in these transmissions. There are lots of upgrade options, you can go with a z-pack from raybestos or I am sure there are plenty of builders that have their own tricks up their sleeves for beefing up the 3-4 clutch. While you are in the trans, I would suggest upgrading the reaction shell they sell one called "the beast" that has worked well for me. Price depends on if you are going to pull it and bring it to a shop or have them R&R it as well. Safe to say probably close to a grand at least to have it done. Others can chime in with their ideas and prices. Make sure you FLUSH your cooler and lines no matter how you decide to do it, either have the tranny shop do it, (most require it on a rebuild anyway) or if you pull it yourself, buy a cooler flush can Lubegard makes one that works reasonably well. DO NOT SKIP that step or you could be rebuilding again before you know it. HTH. If you want any other info lmk. I'll help if I can. Other upgrades to consider would be the 5 planet carriers that can be found in the truck 4l65e transmissions that came in escalades and the silverado ss etc. They also have a 6clutch 3-4 pack that has a bit more holding power. Def. would suggest a shift kit as well when you do the rebuild. They aren't that expensive, and can help firm up the shifts so they slip less under heavy acceleration.