I have a 2007 Toyota 4Runner with 150,000 miles that is in excellent shape but when I get up to 35 to 45 miles per hour it feels like it is bouncing..

Yes, My husband bought new tires a while back that are filled with nitrogen...at first I did not have a problem. about 5000 miles in this problem started. we had new struts put on and tires balanced and rotated. This does not happen when I first leave and the tires are cold but with 2 new front tires today it did it immediately. The front end has been aligned twice..

what brand tires do you have? and when you had them balanced did they road force balance the tires? No matter the brand all tires cant be made perfect. Had a f150 the other day new on the lot that would bounce going down the road took them off and road force balanced them and found the rear two tires had excessive road force and the tires were brand new. sometimes you can find the spot of imbalance and mark it dismnount the tire and move it to a different location on the wheel and it will help. sometimes that will not help. the balancer machine will make that call.

sounds a little far-fetched, but could it be the nitrogen because nitrogen freezes

Thank you kbro..My husband has called someone that can road balance them and I am going to try that...appreciate your help.