Unless anyone else states otherwise, go ahead and do just the two if the other pair on the vehicle still have some good tread on them. It should go without saying, but I will say it anyways, make sure the two you change are onto the same axle (ie. the two tires on the front or the two on the rear, never both lefts or rights). And it's always a good idea to put the two with the most traction on the front so that you always have enough grip to steer properly.
Also, not all tire brands are created equal. The sidewall might say they are the same size, but in reallity, two differing brands of tires will make their tires slightly different in size. This becomes more apparent with the larger tire sizes (ie. a 35" will actually measure 33" on one brand and 34.5" on another). So if your getting just two tires, then try to get the exact same brand of the two you are keeping.
If you still have doubts, then change all four, it doesn't hurt to have some fresh meats under your Rover, then you know you don't have to worry about them for awhile.