The Driveshaft may not be installed correctly..
Remove the 4 bolts retaining it to the Pinion Flange, and turn the driveshaft clock-wise until you've reached the next set of bolt holes..
Reinstall the bolts, and tighten them back down..Not installing the Driveshaft in the exact orientation as it was removed can cause major vibration issues..
That's why you're supposed to mark the rearmost Driveshaft Yoke Flange, and the Differential Pinion Flange before removal...
My other thought was along the same lines as what your dad was thinking..It possibly being a flexplate/ torque converter issue..
After installing the rear main seal..Here's what you should have done..
Before removing the Flexplate..Take note of it's orientation, and mark the plate so when you go to install it's in the same position..
When you install the Flexplate..You should hand tighten all the bolts initially..Then using a Torque Wrench..Torque them to spec in a star pattern..
Torque the bolts to spec in a star pattern is crucial to the Flexplate itself..It's similar to the way you tighten lug nuts on a car..
If you don't tighten them in a star pattern..You risk warping the Flexplate..Same as you would risk warping a Brake Rotor if you do not tighten in a star pattern..
Not only must they be tightened in star pattern, but also torqued to spec..Ok you get the idea...Impact Gun..Bad Idea..
Let's talk about how you installed the Torque Converter on the transmission..
When seating the Torque Converter on a 4R70W you should feel "two" clunk sounds while rotating the Torque Converter..This ensures it's all the way seated..
Once that part is done, and you go to raise the transmission up to line the Torque Converter up to the Flexplate..
After aligning the studs of the Torque Converter up with the holes in the Flexplate..Careful not to unseat the Torque Converter here..
You should be able to freely move the studs with your hand while they are protruding through the flexplate..Any binding here is bad..
Once you've established there is no binding then you can start tightening up your Bellhousing bolts..
After you get all those tightened..Then
you can tighten the Torque Converter to Flexplate nuts down..
Did you do anything differently than the way I described?..If yes..What did you do differently?
I'm sure I missed something..Just trying to help you sort things out..