Sometimes "testing" it like you did will not actually tell you about a clutch problem, because it gets worse when its hot. My clutch was worn out and starting to de-laminate. It was ok when I first started the truck each morning. but after 10 or so miles of driving, it started getting reeally hard to shift. 1st, almost impossible, 2nd-4th, difficult but rev matching made it possible. However when i did the same test as you, i could also spin the driveshaft just fine because everything was cool when i first did that.
The better test is to get it all hot, come to a stop, clutch in, and leave the stick in 1st. keep holding the clutch down but now give it some gas. If your clutch (/the rest of your clutch system) is ok, your truck should not move at all. If it's starting to de-laminate and "flake apart" when hot, you will feel the truck try to roll a little bit.
in fact this can even cause a rough/too-slow idle if you have the habit of leaving it in gear with the clutch held as you sit at stoplights/etc.
The other thing to rule out, is that the clutch hydraulics are ok. make sure both master and slave cylinder are totally bled of air. the only 2 ways that REALLY work are:
1. taking the master, reservoir, and hydraulic line out of the truck (disconnectd from the slave - push in the white plastic clip really really hard with 2 small flathead drivers while pulling on the line)
Remove the 1 8mm sheet metal screw for reservoir, 2x 13mm bolts for the master, use a screwdriver to take off the neutral safety switch and the clip attaching the pushrod to the pedal, and remove a few 5.5mm screws from under the fender liner to make it loose enough to wiggle the master out of the firewall)
Then bench bleed them with the master cylinder held upside down and the slave connection of the line capped off (heres a
good video). Then reinstall all that in the truck, connect the line to the slave, have someone hold the pedal down, open the slave bleeder 1 turn (8mm open end wrench) and pour a full bottle of dot3 through from the reservoir (using some clear tube attached to the bleeder and a clear bottle to watch til the bubbles stop or are at least super fine and sparse). Then close it all up and pump the clutch up 10 or so times.
2. if the system is still /kind of/ working, a vacuum pump with one of those pointy tips shoved down the neck of the clutch resrvoir and pumped for 5-10 minutes until bubbles stop, is the other way.
I just think you should check, since difficulty with 1st gear is a super common sign of problems with the clutch/hydraulic system, like the dealer said. hopfully that is good news. Try the "test" I mentioned and see what happens?
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