Dan: Check your t-case shifter linkage adjustment and/or t-case mounts. The 208 shifter mounts to the floor. When under load the t-case will raise up slightly, especially if the mounts are worn out. If this is happening and the shifter is out of adjustment it may be pulling on the lever on the t-case and trying to go into 4wd when it comes back down under decceleration.
Hank: Has your 203 been converted from part-time? If not, it might be that the chain is loose and slipping when you let off the gas.
Of course there could also be some other problem with either truck such as a worn differential or gears transmitting noise through the driveshaft. Or it could also be worn ujoint(s) making noise.