Rear end feels like it's grabbing or binding under load HELP!

[Albear]

Greetings all and let me start by wishing everyone happy holidays. I purchased a e53 for winter (also a m5 owner). When I purchased it, I knew of the flaws it had at the time. Previous owner had just had it at the shop to replace one of the rear airbag's as it was no good, the pumpkin (rear center section) had grenaded, that's how I got it. (I'm like that, instead of buying a used car for top dollar only to find problems anyway, I prefer to buy one for a good deal that needs a few things so I can go over the whole vehicle and know what I have in the end. So long story short, after a long hunt I found a low km used pumpkin and installed it with all new mounts. I've had to do both front wheel bearings and a few other jobs to it. But the problem I'm still faced with is what feels like a binding up in the rear end once you start getting over say 40-50 km/h.. it cannot do highway speeds without constantly binding and bucking. I've had it up on the hoist and both rear axles are good, no play in the joints, bearings are good, brakes are good etc. I'm at my wits end trying to find the problem.. if I'm accelerating out of say a parking lot it doesn't seem to do it hardly but as soon as I straighten out bump bump bump it's bucking. Throttle response is good, no misfiring or problems there.. transfer shifts perfectly and happen's in manual mode also regardless of gear you're in or rpm range. Could the axles be bad even though they inspect fine? The driveshaft also seems fine no play, guibo is good, same with front driveshaft. Support bearing is also good. I'm out of ideas where to look next and don't want to just throw parts at it or I may wind up building a whole x5 before I'm done..lol. ANY help would be awesome. (No codes in any modules (I'm just trying to relay as much info as I can think of to help the gurus with their assessments)) . All tires are almost identical tread depth. I'm out of ideas.. please help.
Barry