Question regarding A/C condenser fan/fan relay: ever since I purchased this rig, I've noticed the condenser fan only kicks on when temperatures outside and in the engine bay are super hot. Is this normal?
Yup, that's how it works from the factory on your 1999, because the OE "Compressor Pressure Switch" has such high "turn-on" pressure. You could replace that switch with one that operates at lower pressures to effect fan turn-on much sooner / at much lower pressures. This is something I've been planning to do myself on my 1998 but haven't. Read on...
To me, the condenser fan should kick on whenever the A/C is on, regardless of outdoor and engine bay temperatures.
The fan should turn on whenever there's a benefit, e.g., to the AC or engine cooling. When ambient temps are low and the AC's on, for example, the aux cooling fan's hardly necessary as the engine fan's airflow keeps the condenser cool.
Much like I've seen on other vehicles (not GMT400), the aux cooling fan could turn-on once the high-side pressure exceeded 200psi or so, to aid the AC condensing operation.
The aux cooling fan might also turn-on to cool the engine bay after engine shut-off in hot ambient conditions. What's this got to do with AC? A lot. Keeping the engine bay cool during shutdown lowers the pressures within the AC system, which are (of course) affected by the underhood temps. The benefit of keeping the underhood temps low after engine shut-off is that, on subsequent, hot engine start and immediate AC turn-on (a pretty typical occurrence) the AC high-side pressures won't go sky-high. Ask
@PlayingWithTBI about the time his system blew a hose on the high-side after hot start-up and it dumped the contents of the AC system (oil and refrigerant) into his engine bay.