Thank you for the pictures. The axle on your Forest River Flagstaff is a standard axle You can flip your axle to lift your pop up, since you have double-eye leaf springs. Those options are:
Dexter Trailer Springs Over-Under Conversion Kit, part # K71-384-00, for 2-3/8" axles or
Dexter Trailer Springs Over-Under Conversion Kit, part # K71-385-00, for 3" axles.
I have include product description links and video demo links below. view full answer...