After my truck sits for a couple of days I start it and put it in gear and it won't hardly move. It acts like the transmission is slipping. After 45 seconds to a minute it locks up and does just fine. Any one know what is causing this? Is it normal?

Normal. Search for "converter draindown". Fluid in the torque converter leaks past the rotating sealrings over time. Usually takes a few days of non use.

It takes 20 sec for the converter to refill and transmit torque if fully drained (half full).