I need some advise

my 73 D100 had something along the same lines. mine was a 360 np435 8.75 setup. did a new clutch, flywheel, and trans mount. had a HORRIBLE vibration towing under heavy loads. (5000#+) replaced the carrier, helped, wasn't completely right. did leaf spring bushings. solved. my 66 dart, just did a full suspension/disc swap. pinion angle was 3 degrees with the 7.25. swapped in an 8.25, balanced driveshaft, and stronger shackles that weren't almost rusted in half. 1 degree angle. I had a vibration. 30-50 mph. bought shims to increase the pinion angle to 2.5 degrees. fixed. good luck!