That same feeling can be the pinion also
yes this too, but usually easier to find. if you can get under the truck in neutral so there's no drive train load, you can usually feel slop in the pinion by pulling up and down most leak too when going bad too
but it's been my experience no one can change u joints on modern drive shafts without messing them up. and most local shops don't have the level of balancer needed for today's high drive shaft speeds.
back in the days of 3 speed trannys, the factory would put a 2.56 highway gear in, the drive shaft speed would be slow and no issues. today we run big over drives and 3.42 and up gears. spins it's balls off at 80mph. you needs 3rd and 4th order balancing for that. what that even means lol but it used to be Nascar level stuff. but your local redneck drive shaft shop making a living off 4x4 trucks probably doesn't have it.
I've literally been in the back seem the 0 read out on the balancer, installed in the car and rattle your teeth at 70. go to a junk yard, pull one out of a beater knock the dirt off, install and dead smooth. now this was a older car with a manual tranny that's sensitive, but you get the idea. the other thing I've found is the oem has a metal tab across the rear endcaps that go into the yoke. it holds them in during assembly. if that's intact, it's a good sign. if that's broken pass on that used one.
that's just my experience. others somehow hammer in new joints without a 2nd thought and drive on haha