03 Nissan Frontier 4x4-90K. Replaced rear-end due to humming sound/ had a vibration from rear when applying brakes, replaced shoes and drums, 2 days after the brakes, I went on the highway and noticed a vibration at 40 and above. Brought to Firestone and 2 other tire dealers adn was told to replace the original tires. Replaced all 4 and had balanced. Still there, jacked up the rear and ran at vibration speeds, it definately comes from the rear. Cleaned the mounting surface for the rim to the drum, swapped front to rear. Still there, tires balanced.I'm stuck now! Help!