Recent issue i'm having with using 4x4...when engaged in either 4wH or 4WL and moving over 5 mph, there is a hard jolt/thump every 10 seconds or so felt at/near the transfer case and maybe even the front axles. Also feels like after the thump/jolt the car temporarily will pull to one side, then another jolt/thump and normal, and so on. After moving the selector to 2WD, the 4WD feels engaged for about a minute, at which time there is another hard jolt/thump as it releases back to 2WD. Any ideas where to start before i let a Ford dealer tear into it? Thanks in advance for any guidance.