I use my small step stool that I normally place at the cargo doors for getting in and out. I stand on that and use my legs to lift the roof with the roof on the flat of my back. Like doing a squat but easier on the back then having your arms outstretched above you.
__________________
2005 E350 RB 6.0 PSD for extended fun
1989 Landcruiser FJ62 for local fun
2011 VW TDI Golf for hwy fun
Also have back issues and use the cargo bar regularly. I found that lifting the Gaucho bed seat was also causing back strain. I hook a strap around the grab handles and the top of the cargo bar. It raises up nicely until I can set the support leg.