In the scene where the jeeps lose their power, look out Grant's window and you can clearly see by the foliage right next to it that they're nowhere near the t-rex. But obviously, in the next scene they stopped right next to the t-rex.
Grant is sitting in the drivers' seat, which is on the 'left'. The T-Rex paddock is on the 'right'. So when the camera is on Grant, it shows the foliage on the other side of the road to the T-Rex paddock. When the camera is on Malcolm you can see the paddock.
At this point in the story the electricity was still on. Nedry even said "a few of the minor systems might go on or off", the ride being one of them. The power wasn't shut off until Arnold flips off the Main circuit breaker.
The car they are in are not jeeps, they are ford explorers. Get it throgh your thick skull. I hate when people call them jeeps when thay are ford explorers!