Is the letter "d" sometimes pronounced like a glottal stop? For example is the letter "d" in the word "wouldn't" pronounced like a glottal stop?
To my knowledge nobody claims that voiced "d" sound is ever glottalized in English. Strictly speaking you've asked about letter, not sound - in that case, well, you need to things: 1) this particular d is actually pronounced voiceless 2) it's glottalized.
Speaking of specifically "wouldn't" - I never read about this word being D-glottalized. In fact, /d/ in American dialects is indeed quite often indistinguishable from /t/ - both medial t and d both sound as [ɾ] (alveolar flap), which shouldn't be confused with glottal stop. Also, as it said in the link provided, before a syllabic /n̩/ t become glottalized while d never does.
Also this excellent question (and even more excellent answers provided) might give you some food for thoughts.