Haha, it literally ONLY happens in that one spot -- I bet it's just funky!!

 

I had a 2001 F150 and now have a 2007 F150. Both are 4x4 with LS. I had that same problem with both trucks and all they needed was a bottle of XL3 additive in the rear differential. I don't know if that would help or not with non-LS differentials. Another possibility could be low rear differential fluid level. The differential started to leak on my 2007 and it did the same thing.

Another possiblity could be your engine. My 2007 developed a low engine oil pressure issue which seemed to have been caused by sludge buildup. Ford dealer said I needed a new engine, but it appears that has been fixed by performing a BG engine flush. Runs great now. When the engine first started running rough due to the oil pressure issue, it felt like the rear differential again. First thing I did was get under there and check fluid level.

Good luck.

 

Thanks. My gauge shows good oil pressure at all times. I recently had my rear axle seals replaced. Since I didn't perform the rear diff fluid change myself that they did at that time, I could fix it or rule that out by changing it again myself.