Vibration between 45-55 mph 91 2wd shortbed

I have a bad vibration that comes on as I approach 45mph and smooths out around 55 mph. Pretty sure it isn't tire balance since i have swapped tires and the vibration still occurs. I do have slightly ovrsized tires: 235/75 R15 if that makes a difference and the truck has a 3" suspension lift with aftermarket Torsion bars up front. I dropped the driveshaft and inspected the u joints and they were fine. Any suggestions? It seems to be getting steadily worse.

You might want to check your bearing, I had a verbriation in 86 once I hit 55 and up, it was the rear bearings that needed to be replaced.