New camper (to me) that weights about 4200 lbs causes my Dakota to sag pretty bad in the back. Anyone know of or use a load equalizer that isn't too expensive (say under $250) that would work well.

You might want to post this up in the hunting gear section of the general hunting forums. I think a lot more people look there than the classifieds, might get a better response.

IMHO that is a pretty heavy trailer to be hooking up to a Dakota without taxing the engine/transmission. If you haven't done so already, the first thing I would do is go talk to a dealer to see if the vehicle is strong enough to even tow something like that without ruining it even if you can find a decent setup to level it out. I have a Silverado 4x4 shortbed and just started towing a 19' tandem axle trailer 2 years ago that weighs a little more than what you mentioned when it's loaded and it even takes that bigger truck down a little bit, but not enough that I needed to add sway bars or an equalizer. I'm guessing that your Dakota isn't a big enough vehicle to be pulling what you bought, but I may be off base!

I had a couple extra leaf springs put into my old Cherokee for pulling a good-sized pop up. Helped quite a bit and pretty cheap. Just a thought.

Unless you can find a used one somewhere you're not going to get away cheap for load levelers and sway control. And I would definitely not tow a trailer that size without them. And, depending on your model that is pushing the limits of your Dakota.
See this: http://www.new-cars.com/2004/dodge/d...er-towing.html
You can change the year but you can see the towing range varies considerably.