Heather_98burb

Hello all, having issues with a burb and need to figure out why it keeps failing....98 surburban 5.7 4x4 automatic tranny, 1st rebuild lasted 6 months, 2nd rebuild lasted 1 month, 3rd rebuild lasted 488 miles, started going up hill and felt like was slipping, got kindof noisy and let off, pulled off road and went to back up--reverse gone--then went forward very noisy and had towed home....this is basicly the case with each go out and each time it's been tore down the planet gears are chewed up and the rear thrust washer basicly welded to shaft....have took off factory air transmission cooler and replaced with factory line, replacedthe factory radiator (that has cooler), converter replaced everytime,basicly everything gone through....what could be causing this issue? any ideas out there? has the 4L60 tranny with deep pan...
Need help please,
Thank You,
Heather