This video will show you how to replace the internal and external transmission filter along with the fluid on the Allison 1000 transmission found in the GMC Sierra and Chevy Silverado 2500 and 3500 HD trucks. From 20012005 this was a 5 speed transmission and then was upgraded to the 6 speed in 2006. Filters and fluid was purchased from Amazon and I will provide links to the parts below.
GM Internal Filter: https://amzn.to/3DWkGmC
Allison External Filter with Magnet: https://amzn.to/3h6qmlf
Castrol TranSynd 668 Transmission Fluid: https://amzn.to/3t0vDgF