After you replaced the pinion seal, how did you adjust the pinion preload? If you don't know what preload is, then I can tell you what your problem probably is. Remove the driveshaft and try to turn the yoke by hand, if it won't turn easily, the preload is probably too high. If it's a little bit tight, the pinion bearings will loosen up in time. If it's way too tight, the bearings will fail.
Unfortunately, if you've overtightened the pinion nut, it's not a simple matter of loosening the nut to reduce preload. There's a crush tube between the yoke and pinion gear, and once you overshoot the correct preload the only way to reduce it is to replace the crush tube and start all over again.