Disengaging is what the clutch does when you shift gears. You where not getting the clutch full engaged when it was slipping. This can be a worn out clutch but can also be caused by the push rod to the master cylinder not letting the master cylinder piston reach the point where it fully releases the pressure to the slave cylinder. There could possibly be some malfunction in the master cylinder that could cause this but not a typical failure mode of the master cylinder.
If it slips again, check the push rod to the master cylinder to see if it is tight or loose. That may help guide you in the right direction to resolve your problem.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.