4runner won't accelerate in drive...HELP
 

This morning I tried to drive my 2000 4runner to work when the vehicle had a very hard time moving in drive. When I pressed the gas, it was sort of just reving the engine and the 4runner hardly moved. I put it in reverse and it moved normally, but when back in drive, it hardly moves. Is this a transmission problem? Can it be easily fixed myself or should I just bring it to the dealer to look at? Any advice would be great. THANKS!