put the truck in neutral, block the tires, and then turn the driveshaft back and worth, you should have minimal play, maybe 1/4 inch turn, if that, but one thing that makes me wonder is if it were the rear end, wouldn't it just get worse as speed increaes? How are your u-joints? When the u-joints were going bad on my 78 f100, i would get a vibration at higher speeds, and then a thud when you stoped or started to drive. Do you have a 2 peice driveshaft or a single? Could be the carrier bearing if it is a 2 peice.