Anyone else with BAD knees have problem w/lunges?

The part that i have a problem with is not with the lunging knee (keeping it in line with the toes and not going over) but with the back leg, i cannot bend the knee, i have to keep it straight, meaning i have to lunge farther forward with the leading leg. i haven't done lunges since the 80's because i'm just coming off a 15 yr long hiatus from exercise to due severe Chronic Fatigue Syndrome/Fibro but i used to do lunges that way even back then due to bad knees, getting back into weight training recently and all the lunges i see nowadays are with the bent back leg, just cannot do it, it feels really yucky and awful on my knees.

There's nothing wrong with adapting an exercise to make it work for you, better than injuring yourself. I've had to adapt lunges before too because of knee problems. I've found that back lunges are easier on my knees.

I'm not sure I understand exactly what you're describing? Why can't you bend the back knee? Is it knee pain or something else? My first thought is that your hamstrings might just be very tight. Have you tried hamstring stretches to loosen it all up?