Brake hanging up?? or differntial hopping??
 

i recently did a brake job in the rear of my 1985 gmc sierra classic k1500. everything went ok but after the 2nd day i was driving to skool and i got a mile down the road when all of a sudden there was this shaking in the back of my truck. so i turned around and parked it at home. after skool i took it for a drive with a buddy if mine and it felt like a tired wasnt on rite but it was all fine and tight. so i backed it up in reverse a couple times and then it was fine. could that have been caused by the brakes??or differential going? i added fluid to the differential cuz it was rele low. i added almost a whole quart of gear oil.

When you do a rear drum shoe job, after all the installation, you gotta back up is reverse and give it some gas and then hit the brakes hard so they can adjust and set. Do this a few times. You will hear it clicking a couple times. Try that and see if that helps.