Put it up on jack stands, pull the drums off and run it up to 130km and see if it vibrates, just don't hit the brake pedal with the drums off. Just put it in neutral when it slowing down.
I’m tempted but a bit to dangerous for me to pull 130km on jack stands and no brakes.
I discovered something today with this driveshaft.... put it in a driveline shop today to try and help diagnose this issue. After close inspection it appears the phasing is off. The ujoints are a couple degrees off and should be even to each other. Like the driveshaft is twisted or they welded it up a bit off when they built it. They say that would cause a vibration. Then they ran it on the rack and we visually could see a wobble in the shaft. It appears this may be shaft related. The outfit that built the driveshaft will be building a entire new shaft to try and make up for this bs first one. Sadly I’m leaving to work for 2 weeks so when I get back I’ll install the new shaft and put a update here as to if it fixes the vibe. Thanks everyone this I a really cool site