Hi all, hoping someone can help me out. Yesterday I was driving to work down the interstate and all was well to a point. I was coming down a hill from an overpass so let off the accelerator and noticed a slight rumbling sound. Over the next few miles the rumbling sound and a vibration got steadily worse until it had the same sound and feel as if i was driving on coarse rumble strips. This happened driving under load and actually got louder and vibrated harder when i let off the gas, but went away when i got below about 35 mph.
Last night changed the rear u-joint on the rear drive shaft, took it out for a test drive and all was good again. Went to work this morning and it was good, but on the way home it started doing the rumbling vibration again, starting off kind of mild and getting loud and rough over a couple of miles. I stopped to pick up my daughter from camp and when i got back on the road it was fine for the 10 minute drive home getting up to 55 mph. After supper i took it out again and it started the rumbling vibrating again when i got to 35-40 mph, getting worse the faster i went.
i did a bunch of searching on here but have not come across any threads with quite the same description. Has anyone else experienced this or any thoughts on what it might be.

My Blazer is a 2000 LT 4WD 4.3 V6 with 227000 km. I changed the front bearings/hubs last year and this is not the same sound as when the drivers side went bad back then.