pulsating drone

[hfd410]

My truck is a 71 with a 5.3 and TKO600 5spd. It has a pulsating drone at 70-75mph in 5th gear only. At 75 in 4th gear its gone. I have recently repacked front wheel bearings, checked tires (it does it with both sets), confirmed pinion and driveline angle, and replaced rear axles with Moser (driver side had very slight wobble). All bearings and seals done in diff also. Still the pulsating drone at highway spd and only in 5th gear. It also goes away at those speeds if coasting. Any ideas or thoughts appreciated....