The solid mount on the tranny could be your vibration problem. Not so much problem as transmitting what vibration you have. I like solid engine mounts but trannys should have some give.
2 to 5 degrees is good threw the movement of the rear, but if you moved the front of the ladder bar, you have changed your instant center. If you adjust on the rear of the bars, it will keep the same I/C if you adjust both the same.
When looking for a vibration, I jack rear up and first check for loose u joint or sloppy tail shaft or pinion bearing. Then remove wheels and tires, if you have drums a couple washers and the nuts back on and run it. Still have it, get a chalk or grease pencil tape it to a yard stick and prop it on a short jack stand and move it slowly towards the drive shaft, front and back. This will give you a rough idea where it is. Don't forget a bent or twisted axle can do this too.