The 2.8V diodes will work fine with a resistor. Here's how you do it:
Subtract 2.8V from 12V.
12V - 2.8V = 9.2
Divide voltage by the current. Lets assume as an example that the led draws 20Ma (20 milliamps). That's .02 amps.
9.2 / .02 = 460
The nearest common resistor value is 470 ohms, 1/4 or 1/2 watt will do.
Solder the resistor to the long (positive) lead of the led. Led's are sensitive to heat so use a heat sink between the solder point and the led to keep from damaging it.
One particularly good application for Leds in AD trucks is replacing the high beam indicator light. Often the colored lens has been melted by too hot of a bulb having been used in the past. A led works nicely as a replacement. Some leds are two color, so for the high beam application the second color can be used as a turn signal indicator in a position that will be more easily seen without trashing up your dash.
