Here is a way to check out the glow plug relay.
To do a quick test of your glow plug system locate the glow plug relay under the engine cover (plastic hood looking thing on top of the fuel filter, draw a line from center of filter to right headlight and the relay is about 6” from the filter housing under that line). When the truck is having a hard time starting use a jumper cable and jump between the two large terminals for 90 seconds or so. This bypasses the relay - the glowplugs draw a bit of current so be ready for a spark or two. Disconnect the jumper and try to start the truck. If it starts fine then your problem is either the relay or what controls it.
To diagnose the relay further you will need a digital voltmeter. With key off you should have battery voltage to one large terminal and nothing to the other large or either of the small terminals.
With key on for the first two minutes or so you should have battery voltage to both large and one small terminal. The other small terminal should show ground. This shows the relay active.
With the key on and after the glow plug cycle has finished you should have battery voltage to one large and both small terminals. The computer activates the relay by grounding the second small terminal thus activating the relay.
It is possible to have battery voltage to both large terminals when the relay is active and still have a bad relay. If the contacts inside are pitted or corroded you may have enough voltage passing through it to light a test light but not enough to actually run the glow plugs. Generally you will lose voltage across the relay if this has happened. This is why you need a meter (preferably digital so you can test the glow plugs if you need to) rather than a test light to check the relay. With the glow plugs on (first minute or so after turning the key on in cooler weather) and the engine not running check the voltage at both of the large posts on the relay. There should be no more than .3 volts difference (less is better). You also should not have significant voltage loss from the battery + terminal to the always hot terminal on the right inner fender mounted starter relay, and again at the always hot glow plug relay terminal. Check this voltage loss with the key on, glow plug relay active. You will have to cycle the key a couple times to do all the testing unless you can get to all three terminals in @ 90 seconds or so.
