The rear end vibration when you go 50 or more miles an hour

Sounds liike a ring and pinion on rear differential. U can pull the cover off and check this or another thing would be to check your u-joints.

if the wheel alignment is faulty ( castor wobble ) at 70 mph there is no way that you can describe it as a vibration as it virtually rip the steering wheel out of your hands. If it is a torque converter problem then at around 3850-4025 rpm stationary there will be a vibration of the vehicle and engine The sort of vibration that you are experiencing at 70 mph will come from a drive line problem. If rear wheel drive then it could be faulty "U" joints, worn slip joint or failed rear engine mount
If front wheel drive it may be cv joints or bent drive shafts,wheel bearings or brake callipers not moving freely on the mounting bolts.

IF you have rear wheel drive then check the u-joints for wear/loose. Check slip joint for wear . Check rear engine mount is in good condition. Check wheel balancing

Did you service the caliper slide pins to ensure the caliper slides freely? A rusted pin causes vibration.

Also check your ball joints and ties rod ends for wear. New bigger rims can accentuate worn components.

Do your new rims include hub adapter rings to EXACTLY center the rims on the hubs?

tires out of balence

check the axle shaft assy,an excessive joint angle due to incorrect toe-in or trim height,,,,,Wm or damage inboared or out boared joint,,,,,,,A sticking spider ( tri pot joint)

wheel bearing

Check wheel balance,
check steering linkages for loosness - such as tie rod ends, steering rack linkages etc,
check driveline for worn/loose joints and bearings.

rear end differrential....have it checked

Sounds like front differential or front drive axle issues. Check the front differential gear oil then take er' to the dealership before you end up spending a bunch of $. Most people change their oil frequently in the engine but forget about flushing the transmission fluid and changing oil in the front and rear differentials. I have three Ford 4X4s. Two which really get used hard. The 93 has 311K miles and still runs excellent. Trans and differentials get a fluid change every 25K miles and moble one synthetic engine oil changes every 5k miles.