I've never used the Parallax receiver and transmitter so there may be reasons why the things I've learned with the simpler (and order of magnitude cheaper) TWS transmitters and receivers may not all be applicable.

From the LADJ excerpt that was quoted, shorting the resistor so that Vcc is applied to LADJ will give maximum output power from the transmitter.

The distance above the ground will also have a big effect on range. Reflections/refractions from the ground, buildings, trees, etc. degrade the signal. Most of the range estimates that accompany these devices are outdoor, line-of-sight figures. Indoors, my rule of thumb estimate is 20-25% of the outdoor figure.

An efficient, directional antenna (e.g. a yagi) on both the transmitter and receiver can give you large increases in range by concentrating the power in one direction rather than spreading it equally in all directions. The stubby antennas on the Parallax modules are not very efficient and are omni-directional. There are some yagi antennas for this frequency that are printed on circuit boards (or even mylar). Here's an example...A wideband preamp between antenna and receiver can also give a big increase in range but this may not work with all receiver designs. In some cases, the increased signal can overwhelm a receiver but I've used them successfully with the TWS receivers and have even tested with 2-3 of them cascaded. This has one major advantage - it does not violate any of the legal limits on transmitted power levels.