Heavy vibrations

[Cdubvw]

Hello, seeking some knowledge from some of the c10 masters! I am having a problem where I am getting a bad shaking/vibration. It the feeling is not in the steering wheel and feels to come from the rear but i may be wrong. So far I have been throwing money at it to try and solve it. I have installed all new u joints, carrier bearing and had it balanced. New tail shaft housing bushing as there was a bit of play. Had new tires put on and balanced twice. Transmission mount is in tact and seems ok. This happens like clockworks at 2300 rpms only in 3rd gear. If I accelerate hard and get up to 3500 rpms I can feel it shake at 2300 as the rpms drop. It only does it in gear and only in 3rd. Thanks for the help.