Driveline thumping/clunking
 

Whenever I have 4-5 people in the car and accelerate or drive up a slope, I get a continuos thump from I guess the drive axle. Car has driven less than 100k miles

My old 745 did the same-it's the driveshaft centre support bearing or the rubber carrier that it sits in. When the car is loaded, the rear axle sits higher against the body of the car, which changes the operational angle of the driveshaft, moving it also closer to the body of the car. If the DCSB is gone, the whole assembly isn't sufficiently supported and will generate a clunking/thumping sound.

In my experience, this is worse in first gear and worse going uphill.

It's a relatively simple repair but made much easier if you can get the car in the air.

What rear suspension do you have on yours, is it Nivos or normal?

Either way, weak rear suspension can allow the back end to sit lower causing vibration not only in the prop centre bearing as Phil mentions but also the diff nose bearing - it's all to do with things not being lined up as they were designed to be.

Several people who have lowered their cars get this problem too.

If you have Nivos, i'd suggest new rear springs and shock absorbers/dampers would also help, if you have the standard set up, look to see if the back of the car is lower than the front when unloaded and on level ground. If the back is lower, renew the springs and shocks.

I have standard shocks, and they have just been replaced. The car doesn't sit noticeably low.

Rear axle movement
 

The high torque on the drive shaft in first gear not only turns the wheels, but also wants to turn the axle in the opposite direction, as a reaction force. There are two torque rods to counteract this axle rotation. With age, the rubber bushings of these rods get more flexible, allowing the axle to rotate a bit further. This causes the drive flange on the nose of the diffirential to lift higher, resulting in an increased propo shaft angle. What you actually feel are the u-joints rotating out of alignement.
Replacing the rubber bushings would be the ultimate solution, but putting shims underneath the prop shaft bearing can also help.

Educational action views of the rear axle: https://www.youtube.com/watch?v=OFlZZ96poIM&t=6s
It is a known "problem" and you can also do a search on Volvo TSB 45/102 for more info.