Posible Transmission problem

Latley when I step on the gas the RPM's go up but the van does not move. I finally starts to move when the RPM's get to about 3500. I am thinking that the trans is just slipping. I do not have any ideas, this van is not worth enough to have it diagnosed by a transmission repair shop. I got an estimate and they said it was not electrical and it would cost $350 for them to take it apart and tell me what is wrong with it. They did not say how much to put the transmission back if I did not want to fix it.

Any ideas would be great.
thanks in advance

Check your fluild level at a full service gas station it may just be low. Its also free.