I just recently got my 260 started after a long engine transplant, and its engine revs great to 5.5K, runs smooth. However, when I get it on the road and up to around 45, it starts to feel like by back end is going to vibrate apart the faster I try to go. I have new suspension all around as well as bushings. I've had an alignment on the wheels too.

I also have a new exhaust too, which I'm suspecting could be the cause, since it seems to clang and hit the mid section when the idle is low, but that stops after the engine is over 1K rpm.

Is there something else I should suspect or look into? I did swap the transmission to a 81ZX 5spd on the same driveshaft from before. The diff is still the same orignal one too.

Just for grins, check your driveshaft's bolt down to the differential. While you're there, check your half-shafts. That amount of vibration isn't unheard of when any one of these three get loose and are off-center to the rotation they're supposed to have.

2¢

Enrique

well just a little update, the driveshaft did crap out on me at school. Trying to leave at 12AM, and thunk. Not a good thing to happen, but luckily for me it was at 5mph. I got a ride back to my place, got a jack and some bolts, and by 1:30 was back at home with it in the garage for a real fix.

I could only find one bolt stub where it broke, don't know when or where the other ones went...