Leaning forwards on squats going straight-leg on deads

[texasmax]

All-

I've been doing the Rippetoe program for 4 weeks or so now- and my squats hit a ceiling that I just can't get out of- and I'm starting to lean frontwards starting from the bottom (changed to A2G to prevent straining my lower back too hard, which needs to catch up). It feels like I'm working out my toes. I'll catch my balance about halfway up, but I can't start the ascent without leaning forward (and I'm doing it almost as a reflex, not on purpose). Id I force myself to not lean frontwards, I won't make it up. Is this weakness in hams/glutes? I'm guessing it is, as recently my deads have me going straight-leg without thinking about it, assuming this shifts the work to my back instead of on hams/glutes.

Should I keep my squat and dead weights where they are, or even lower them, and spend some isolation time working on hams/quads? Lying leg curls? I don't have a piece of equipment I can do GHRs on-any other good movements I can be doing here?

Thanks!!!