Well...from what you describe, my money is still on the C-beam joint, probably the front (transmission joint) one. But, to find out for sure, I'd have to remove the C-beam and inspect the contact regions for scuffing, and/or distortion of the bolt holes, i.e., elongated holes, AND/OR a ridge around the edges of the steels washers is the dead giveaway the joint is slipping!
NOTE: Also, if the rear C-beam joint is slipping, the rear-end tends to kick out to the right under hard acceleration as mine also did. The cure for both for me was/is Boudreau's C-beam plates and some blue loctite to enhance friction between the mating surfaces (ref Bill Boudreau's ZFdoc.com site).
While the C-beam is removed, the motor mounts can be tested by attempting to rotate the motor from side to side by applying pressure to one side and then the other with a (bottle) jack. Any movement can be explored to assure it is only the rubber flexing, and NOT a securing bolt(s) or broken parts at fault.
Keep in touch!
