[LH Lead-Foot]

"Bigmoe"
The best way to think about pinion angles (with one piece driveshafts) is to imagine a straight line going from the center of the front pulley bolt and out the centre of the trans output shaft and continuing to the back of the truck.
Now imagine a line coming straight out the rearend forward. Those two lines should run parralell.

The visual provided by "Bigmoe" is a good one. And the use of the angle meter is a good one.

Don't worry about lifting the trans a little as long as you have daylight between parts and the floor. Use a block of wood during the lift.

I have lost track, but if the trans is a 1999 or newer 4L60e w/3 piece housing, these are about 3/4" longer and maybe causing the driveshaft not going home. Depending on many factors for engine placement, but many mounts allow forward or backward movement. This could keep you out of a driveshaft shop.

As for brake light issues, the duel filament bulb "1157" has been issues for years as the bulb is installed wrong, filament breaks then and touching the other wire, you get feed back, but you said the backup light came on. Right? So this will not be your problem. Testing at bake up light switch is not hard and is next. (Note: Grab some 3/8" extensions from your tool box, place an assorted length of these, between the seat & brake pedal, turning on brake switch, to operated / checking brake lights when alone)

The 67-72 pickup shares wire color going forward, but "BRN" is tail light circuit, LT/GRN is backup lights, DRK GRN is right rear, "YEL" is left brake light. On a manual trans, they used a simple "Ball Detent Switch", but the automatic is switched at the steering columns "Neutral / Safety Switch which has "Rev" switch built into it. I know there are two different part numbers for this column mounted switch, found at the bottom, on top. One 1/4" bolt allows it to be lifted, other end is tab "A" into slot "B" (If I remember right), then remove an inspect. The tab cane broken off, stuck in the reverse position. When removed, you will find using a finger carefully, you allows you to feel a hole in the shift tube shaft if equipped. The tab of this "N/S" switch fits into this and is easy to break off if the steering wheel is moving.
Remove connector, turn key on, check for voltage. The DRK/GRN comes from the fuse box, but the LT/GRN goes thru a firewall grommet and then along the frame to the lights, left side.

This is just one of those things you runs into. I used a console shifter and build my own console with PVC board, but fabricated my own Park/Neutral/ Reverse/ switch assembly using micro-switches.

As for artificial joints, knees & hips are money. Elbow, not so much as there is 3 bones, size and correct angle. Plus, all 3 bone a smaller in diameter, leaving less contact area. Joint failures come loose as the bone is to grow into the "Stippling" of the insert anchors. It's like a knurled tool surface or a file. Depending on use, it can cause the bone to break free and become loose. So, it's easy to see why some Dr. shy away from "That" procedure. Yes, this is the second Dr. But just this month...last guy was 5-6 years ago. It sucks, oh, 6'4".

I can't walk after bending over to wash dishes or shave. Pulled all lumbar muscles in 1987, which lead to bone fusion S1, L5 & L4 in 88. Then in 2005 had the L4 included. Now a decade later, live nerves in scar tissue and burning like a propane torch is behind me. Getting worse every year. Last RFA was August 2017. No luck, want 50%, but got nothing except a bill for new truck radials tires.
I can take 1 pain Rx, wait 20 minutes and go for about 30-45 minutes max, just to rake pine needles. I am up to just under 60 grains of morphine per 24 hours. Anything more then I get a visit from the Feds and have to wear a bracelet and carry a numbered card. My Dr. wants to retire...I ask him to wait 3 years. But TMI - This is my problem now.
Lets get yours fixed.

Driveshaft length is important with suspension and other factor involved. You have to leave room at the slip joint for potholes, not "Duke of Hazards". Your final U-Joint angle should never be less than 0.5 degrees (Stuck in my mind) and no more than 1.5 degrees. Last driveshaft I had done cost $20. Last on I did with 4.5" cut-off, cleanup rear companion flange, Clean deep MIG weld and used two worm-clamps together at rear (Note position of OEM weights), then rotate apart a little at a time, until axle supported vehicle, has no noticeable vibration at 20 MPH, 40, then 60 with hand on fender, forefinger up in the air. It's a procedure in the GM service manual.