So, I know you can say "l’uomo" and that means the man. Can you say "l’donna" to say the woman instead of "la donna" or is the l' in front of uomo exclusively masculine?
It's not a matter of masculine vs feminine. The truncated form l' of the definite article can be used only for words starting with vowel. Therefore l'uomo is correct but l'donna is not.
One point that might be the cause of the confusion is that the sound /w/ (as well as the sound /j/ in words like ieri) at the beginning of a word is considered a vowel in Italian, while it is considered a consonant in English, hence in Italian we say l'uomo but in English we say a war (these sounds are often called semivowels or semiconsonants to emphasize their intermediate state).