1998 Chevy S-10 A/C low side shows high pressure

What are the readings? Are the gauges you are using calibrated? If the A/C blows cold, why are you concerned, if you don't mind me asking.

The device you mention is not familiar to me and it may be telling you that there may be a restriction somewhere in the system, perhaps the receiver/drier is starting to get a bit old(?) I would start with a visual inspection of the system for possible damage or perhaps check for condensation at an inappropriate location which might indicate a restriction. I would also tend to think that since the system seems to be operating properly at this time, you may wish to put dial gauges on it and confirm the low and the high side pressures before taking any action, my dad always said "if it aint broke, dont fix it!" And to respond to the concern about possible damage to the system, there are safety switches that shut the system down if the high side pressure gets too high so that damage won't occur. I have always been a bit suspicious of using just the low side pressures to diagnose the health of the system, maybe the readings you see from the truck are close to the measuring devices threshold and it is possibly giving you incorrect information, just my opinion.