Firstly, some hamstring stuff...
http://www.youtube.com/watch?v=Ol0r8lUqB-Q&list=PLE60F0D16C69CE165&feature=share
Look at the bit about two hip joints - this will become apparent when you watch back your RDL video.
The video on snatch prep is fun because it will focus you on that bottom position in the squat - particularly the front squat - and controlling you pelvis and knees.
Regarding your two videos...
RDL: You're starting off lordotic in your lumbar here which will cause you bracing problems as you descend through the eccentric element of the RDL, as this part of your spine will not suitably rigid. This is where the exercises below will help...
Front squat: nice. Very nice. Now put some more on the bar.
In all seriousness, you are starting from the lordotic position in your lumbar spine again which might not ruin you at the weights you're playing with at the moment (as you are still selling yourself short on these given how easy that set looked), but as that weight increases you will begin to flex around that region causing lumbar pain as it tries to compensate and manage the spinal flexion.
So...
1) Barbell roll-outs, concentrating on your hips in posterior rotation;
2) Planks, concentrating on your hips in posterior rotation;
3) Hip pop-ups, concentrating on keeping your hips in posterior rotation.
Here is a (not quite completely rubbish, but it's the only one I can find quickly) vid of hip pop ups, an interesting test one my coaches had me do right at the start of my lifting 'career'...
EDIT: The reason most videos of these suck is because they're actually called glute bridges (thank you, TheCenturion!)... searching for glute bridge videos is so much better. No prizes for guessing why.
However, the point here is not to do this with one leg, but using both. The real issue is actually making sure your lumbar spine is in the 'neutral' position. So assume the start position (heels as close to your bum as you can) and get your hips/lumbar into neutral, which is where you can just about get your fingers into the space between the floor and your lumber spine. This might require a bit of what appears to some dubious hip-centric movement, but it's important - I assure you.
Whilst holding this position, brace and drive your heels into the mat (imagine trying to kick yourself in the back of the head), squeezing your glutes. Then back down and re-check your lumbar position: can you still only just about get your fingers between the floor and your lumbar?
If this remains unchanged, that's good: keep going and remember this position when you start any lift, movement or even just walking. If not, keep at it.
Hope that helps!