I have 97 Windstar, 3.8L in which the AC has quit working. I measured the pressure at 70 psi on the low side which is way too high. Any ideas on what could be causing this, or what to check?

Has it been charged recently? If the compressor is operating and circulating the refrigerant, 70lbs is way too high and either there is a restriction in the system, like a clogged orriface or the system has been overcharged. What is the cooresponding hi side pressure? with low at 70, the high is probably dangerously high at 80+ ambient temp if it is overcharged.
Are the suction and pressure lines hot or cold? the liquid line(hi side) should be too hot to touch while compressor is operating and the suction (low side) should be cold and forming water condensation when operating.

Problem Solved: It was the AC cycling switch.