don't be so quick to condemn the oem driveshaft-especially if you don't have gears. At $260 a pop, an aluminum driveshaft is now on my list of "not a chance in hell" (actually I already have one)
I agree, check the u joints, HOWEVER,
it is common for bolts to work themselves loose. A very good possibility is that your driveshaft bolts have worked themselves loose. Verify they are still tight. (they need 85 foot pounds of torque, yet most use a 12 mm wrench to put them on, I sure as hell can't apply 85 foot pounds of torque with a six inch wrench!!)
I also had a thunking once, and my trans-bell bolts had worked themselves loose. Verify all trans to bell and bell to engine bolts are tight, verify the condition of your control arm bushings, and your torque boxes, and you'll find the source.
also worth inspecting is the rear wheel bearings. jack up the rear, and take the wheel off, grab ahold of the axle shaft by the lugs and move it around. Excessive play means your wheel bearings are shot.