My car just began to "shake" when i accelerate in 4th or 5th gears... any idea what might be the cause?

the best way I can describe it is it the car shakes just like when brakes are goign real bad and it shakes when you hit the brakes.. but its only doing it when accelerating in 4th and 5th gears...

 

I had a shake like that, mine ended up being an inner CV joint about to fail. Shortly after the shake it began to slip... No LSD so once it would slip it was like I was in neutral. Luckily it got me home. Give your driveaxles a look see.

If it wasn't under acceleration I would say get your tires re-balanced.