With a leg press or hip slide (different versions of each) machine the lower back is being supported. With the squat it is not, so more muscle response will be brought into play. With balance, another factor, the back is receiving more work all the way up to the shoulders when squatting. Including the hams, which help a great deal. Reason why the squat is just about the best single thigh exercise for full development.
Guessing the "back pain pump" is a muscle seizure/cramp/spasm, which may suggest your squatting form needs improvement. Or a nerve or muscle injury problem. A simple longer warmup/stretch, massage or a heating lotion, before squatting, can help.
600lbs (including the actual sled?) may be enough, assuming your going all the way down to rock bottom. Might try the one & one half rep system. There are two versions of this.
1) Start from the bottom position, do a full rep and than only come half way down and than up again to the top position. Than drop down again to the bottom original starting position. That's considered one rep.
2)The other version is starting the rep from the top, go all the way down, return to the half way mark and than drop back all the way down again. From there return to the top starting position.
Take your pick. Either versions requires a slower rep and a range between 8-10. Another example of TUT, the burn will be felt quite well. As will the pump. Sometimes you can make an exercise much harder and demanding, with a lighter work load.
The one & one half's works well with most any other muscle area. Just another option in BB'ing training to explore, if anyone wishes too. Good Luck.