While sifting through the mounds of info in the 2nd gen thread about their front diff vibrations, I noticed that there are several 1st gens with similar issues (myself included). I made this thread to compile our own experiences and make it easier for us to filter through the information.
Please post your 1ST GEN experiences with diagnosing your problem, replacing the bearing and if it was on the driver or passenger side, what manufacturer you used (ECGS or OEM), details of the job, and whether the problem was fixed or if it came back.
Problem: (Language taken from 2nd gen thread, thanks little brothers)
It has been identified in a number of previous threads that vibrations may originate from a worn needle bearing and/or CV axle shaft on the driver's side of the front diff, a condition that is often exacerbated by putting a lift on the truck. A worn bearing/axle shaft condition may be present if you experience vibrations/groaning/growling (most typically between 30-40 mph, but also at other speeds) and the vibrations disappear the instant you switch from H2 to H4. Therefore, this is not a place to discuss rear driveline vibration issues, rear carrier bearings, rear driveshafts, axle shims, or other irrelevant matters.
Replace the front differential needle bearing, whether it's from ECGS or OEM. This is the solution for 2nd gens and hopeful to hear similar experiences for 1ST GENS.