Hi, Cheryl!
Ouch! Neck disc replacement! That's gotta be a challenge . . .
As an alternative with your body bar for weighted squats and plie squats, consider holding the bar at the front of the neck and across the front of the shoulders, gripping the bar with an overhand grip and hands placed together, elbows pointing forward. Cathe demonstrates and uses this grip along with certain of her crew in Boot Camp in at least one set of plie squats. IMHO that could be a good bar placement for regular squats as well.
Not so much for lunges. Simply use two dumbbells whose combined weightload equals whatever you might use for a barbell, and let the dumbbells hang naturally down by your sides, palms facing the sides of your thighs. Something, however, to keep in mind is that, the heavier the dumbbells, the more you might experience hand, forearm and shoulder muscle strain if you do lengthy sets of lunges, so just pay attention to that and rest if you need to.
In the Slow and Heavy series, Cathe also does a pre-exhaust set or two for legs with squat and/or plie squats holding one really heavy dumbbell (I think her weightload is 40 lbs) in BOTH hands right in front of the body near the pelvic area. I think you could hold it sideways with both hands, or hold only one side of it in both hands with the dumbbell hanging down. OR, you could hold two dumbbells of equal combined weight in both hands, pressed together in front of you.
Hope this helps -
A-jock