So, picked up an s2k after a 2-wheeled mishap that left me needing another toy till I'm healed up. Replaced already a good bit, now yet another issue arises. There has been the ABS light on since I purchased the car. I figure it's simply one of the wheel sensors needing to be cleaned up or replaced. Any way, been busy with school and work so unfortunately haven't gotten to that quite yet, looking to tomorrow. There's been a strange thumping that grows in intensity under heavier braking. It seems to kick in sooner than what I feel ABS would be. Clutch in, moderate to heavy braking, thumping (best description I have) starts to increase in intensity and feels like it's coming from directly behind or underneath the driver's seat. Lifted the car to take a quick look the other day, and noticed some play in the axle assembly on the driver's side in comparison to the passenger side. The passenger's side appears to have already been replaced and is slightly larger diameter compared to the driver's side. Brake pads appear to be in good enough condition to not be contributing to the thumping. There is a small yet apparent leak from the rear differential housing that I'll also be addressing asap. What I'm hoping for is any professional opinions as to what might be the most likely cause to the shuddering and thumping during braking. My best guess is the axle assembly having play is the culprit. Any other things I might be missing or might be a common thing among the S2k's?

Thank you so much ahead of time!