[crazypete]

I vote crossmembers. That bangs under hard acceleration if they are worn or rotted and bounce around on the mounting bolt. Failing that....

I think the rear suspension is moving under load (obviously) but it is contacting something. Your 1rst and 2nd gear only comment means it's definatley about twisting under load. Check the driveshaft and look for scratches where it goes near the 2 mufflers. In mine, there is less than 1/2 inch of clearance at one point. If not, check the pinion snubber at the top of the axle snout. Is it there? If it is, is it rubber or an aftermarket piece. The hardened AM ones do make a little thump when they hit but you really need to punch it to get that happening and the suspension needs to be really low. Check the rear wells for rubbing signs but I dont suspect tire contact too much. If the tailpipes hit, you'd see a shiny spot where they hit. Get a 300 watt shoplight you see 300% more when you use the high output lights! Wedge some rubber on either side of the tailpipe where it goes near the fueltank. Then they wont move at all. Then go and race around some to try and get that sound happening.

If all else fails, put the back end on jackstands with the axel drooping and then jack up one end of the axel and then the other and see if it pops or moves freely. I suspect one of the swaybar mounts or control arms is binding. Finally, if this fails, while you got it up there, pop the diferential cover off and take a good look at the gears. Broken gears whine...loud! So you shouldnt find anything in there. But why not look? Replace the fluid with redline synthetic while you're in there.

=)

Good luck!