This is what I was going to suggest. Either switch out the truck on the rear to be a RKP or TKP until you get used to the DKP on the front and the added wobbles from that, or get extremely hard bushings and tighten the crap out of the back for now.
Personally, I’m still getting used to having essentially a DKP on my front that actually works and while I’m not getting any speed wobbles I can still feel the extra muscles I’ve been having to use on my back leg compared to when I had the defective part. Even just small movements on my back leg can turn my front end so I’m constantly having to fight against any movements caused by bumps or rocks in the road on my rear truck.
Dkps are crap above 25mph, as already mentioned they are for carving below said speed. Would not recommend having a tkp in the rear either as the front of the board would ride much higher than the back. If you must go fast, you will be much happier with rkps. Still recommend better bushings like others have suggested if you want to keep those trucks and maybe change the motor pulley and/or wheel pulley to lower top speed and raise torque.
Depending how wide your trucks is also a key factor. More wider your trucks are, the more stable the board is, I got Caliber III trucks on my board and can cruise 32mph easily with 0 speed wobbles. Try playing around with the tightness of the kingpin bolts, if you get them decently tight it should be a little easier to control. Just all about finding that sweet spot, its weird as well cuz the faster you go, even tho the kingpin bolts are tight, the turning is easier, maybe try that if you haven’t or the other option is to get harder bushings
