vibration in rear part of car

hello friends
i need help once again. i have a vibration coming from the rear part of the car that has been around for a while and i think it is time to tackle it now.
the vibration becomes most extreme over 75 mph with throtel and while coasting it is extreme down to 45 mph. up hill it is not as noticable but down hill it is worse. it has no rhythm to it, but it just gets more extreme as the mph increases.do you have any suggestions or test idea's that i can try to to narrow down the issue?
i would like to go faster than 75 mph and be able to listen to nothing but the engine. any ideas would be appreciated
drive tran is a dana 60 with a detroit locker, worn out leaf springs, cal tracks,and a tko 600

Check 2 things first.. tire balance and the U-joints..
sounds more like a U-joint for the mph on decell

Way out pinion angle does the same thing.

Make sure the drive shaft is the correct length.

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Way out pinion angle does the same thing.

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how do i know if i have the right pinion angle? how do i measure it and correct it if it is off?
i was just going to ask about pinion angle because i just read the post above mine about shims for pinion angle.
would the worn out leaf springs have anything to do with the pinion angle?