Is it 4WD? If so, check the CV shafts where they meet with something called an "intermediate" shaft right at the FRONT differential. I know you say it feels like it's coming from the rear, but if that spline is worn (at the inboard end of both CV shafts) it can cause some weird vibes. I chased a similar issue with mine for months, with lots of parts changed, until figuring it out. If it is indeed 4WD, just crawl under the front bumper and grab the inner CV joint and give it a wiggle. There should be no slop...