When I put it in 4 high my truck kinda "hops" down the road when I put my foot in it a little. Never did that to me before until recently. 4 low works great.. not sure what my problem may be.. any suggestions? Thanks!

 

On what sort of surface? If on pavement, that is somewhat normal, especially if turning. One test, unbolt rear drive shaft and then try 4H with the front hubs locked to see if the front drive works fine by itself. Could be a difference in tire heights front and rear, same brand/size tires and same inflation pressure?