Transmission slipping

I purchased the truck listed above zr2 4.3L with automatic 4L60E transmission a while back. It had a problem with the gears slipping. Mainly it was slipping from first into second gear at first. The four wheel drive also does not work. It only slips when the truck is warming up. It runs smooth when cold. I replaced the TCC solenoid and filter but it did no good. I also noticed blow back from the transmission dipstick while checking the fluid a couple of times. Hence, the fluid was higher than normal when hot. Eventually my clutches failed and I lost reverse. I dropped the pain and as I figured there was metal in the bottom. So I purchased a rebuilt transmission from the Chevrolet dealership and installed it with no problems. And guess what. It has the same slip. Not as bad as before, like when my clutches were starting to go, but you can tell it is the same problem. And it is a completely different transmission. I am not sure where to go from here. Should I go ahead and drive it off the grand canyon? Maybe the inter-cooler? Any help would be appreciated! Thanks