Vibration at 40-50 MPH 1500 to 1800 rpm

Last summer the drivers side rear lower control arm broke. Since the year before I rebuilt the whole front end I figured it was time to the rear. I ordered 2 lower control arms, upper control arm, rear springs and shocks. Naturally all of the parts did not come in at the same time. With the parts I got I changed out the broken control arm, both springs, and shocks. After doing that everything worked fine.

Then the other lower and upper arms arrived. I replaced both of those. Now while driving it I get an intense vibration that had been happening between 35 - 45 mpg and 1200 to 1500 rpm.

I double checked myself and torqued all of the bolts again while it was on the ground (not easy to do). No help on that one.

One thing I did not get right was installing the exhaust again. The rusted studs needed a few washers to tighten it. It was still loose on the first attempt so I took it back off and added another washer to space out the nut. That got it tight but had the undesirable affect a changing when the vibration happens. It now happens between 40-50 mph and 1500 to 1800 rpm.

Researching the issue here lead me to a number of posts that point to the universal joints as the issue. They are cheap so I just bought 2 new ones planning on changing them in a few weeks. Well its now a few months on and I finally took the drive shaft off. The existing universal joints are free with no apparent wear on them. I did not change them. They don't seem to be the issue.

Any other ideas on what might cause this.

As the vibration changed with the exhaust work I'd look at that first, check it's not touching the body anywhere, sometimes old rubber hangers get stretched and let it move around, check for worn engine mounts too.