You may need those springs since your trailer will be near maximum capacity of your FJ. Jayco shows that trailer at 3665# empty, max weight 4950#, tongue weight empty 400#.
The FJ specs are 5000# max tow and 500# tongue weight. These type of SUVs have pretty soft suspensions, I have a Ford Expedition that is rated near 9000#, last summer bought a trailer that is under 4500# and was surprised how much the rear end squatted.
My front lifted 1-3/4" and the rear was down almost 2", by adding a WDH I got it back to where it should be. I would start out with the WDH and see it that works before messing with the springs.