Can post nasal drip cause me to choke in sleep and wake up expelling a lot of clear sputum?

Highly unusual: I suspect that you may suffering from reflux. Need to see your doctor about this. In the meanwhile, trying sleeping on a 4-6 inch wedge to keep your head elevated when sleeping.

Yes and: so can GE reflux, sleep apnea, seizures during sleep, aspiration, cardiac irregularity and a lot of other situations. Post nasal drip is usually not that severe but it is possible. Check with your doctor if it persists.