REM sleep is essential for the restoration of the brain's electro-physiology and -chemistry. The only real ways to avoid REM are insomnia and having a trustworthy companion wake you up every 40-50 minutes. Having experienced both, I wouldn't recommend either. I'm not sure which of Dement's many studies you're referring to (as far as I know, all his studies implicate the necessity of all 5 stages), but basic physiological psychology implies immense repercussions from substandard sleep cycles, especially lack of REM sleep. I would highly recommend taking care of the PTSD symptoms with a licensed therapist in a manner that doesn't interrupt your circadian rhythms.

I hope you have luck taking care of the nightmares!