Thumping sound when car slows down/downshifts(automatic)
 

Any ideas what could be going on, the truck runs fine except for this sound it started making when it lowers speed.

always check fluids first. if you have coolant discoloration,could be radiator/tranny problem . check quick!!

Check your cross member bushing.

Ahh the famous "crossmember busing issue!" Thats what it sound like to me as well!