Took out tranny and put it back in now truck won't start.acks like it bound up

Just took the transmission out and then put it back.... WHY?

Hello, Yes, why? Also, did you have resistance to mating the engine and transmission, or did it glide into place with easy? If the transmission was installed incorrectly, there will be an extremely heavy load on the the thrust bearings in the engine, and the front pump/input shaft on the transmission. This would replicate your conditions. Also, if the linkage is unadjusted, or the transmission is in gear, that would replicate your symptoms. Try disconnecting the shift linkage, and manually moving the transmission to park...