Martin's Comet measured 2.5:1 across a very wide band. We tried a ground wire from the base of the antenna to the frame with no affect. He has his antenna mounted on the right fender, which strikes me as a pretty decent place. We tried Nakman's Diamond, measured the same. I was using my MFJ-269 analyzer, which is maybe not lab quality but works fine enough.
He was using a pre-terminated SO-239 mount, so I wasn't too concerned that he had a bad PL-259. It measured DC fine.
Are the connectors and terminations good and clean? How and at what power are you measuring SWR? Is your truck in the open, away from buildings and metal (importantly metal)?
Very low SWR is generally a good thing to shoot for, but that is not the only criteria for antenna performance. Sometimes things are not what they seem. None-the-less, here I think it's telling you something is not happy.
First, the SBB-1 is a 1/4λ on 2m and the SG-7900 is a 7/8λ on 2m. The SBB-1 very much relies on a ground plane to operate, while the SG-7900 is probably less dependent. This would be my main guess as to why you have a high SWR, a bad RF return. RF is a physical thing and it wants a complete circuit, just like a headlight in your truck or an outlet in your house. So the antenna itself is 1/2 of the required parts, the RF that excites free space electrons needs a way to get back home to the ground of the radio, which is why a ground plane or antenna ground are critical.
A photo would help!
I'm picturing that you have a mount that is a small tab with the NMO or SO-239, where the antenna screws on. The tab is connected to the roof rack, but both are painted and there is not real good conductivity. Further, the rack itself is painted and electrically isolated from the body of the truck.
My guess is scraping some paint from the mount and a length of 1/2" braid running to a /good/ ground (e.g the frame) will fix your SWR. You could ground the strap (NOT WIRE, use proper braid) to the body of the truck and the body to the frame in a couple of spots, if you want.
It's also conceivable that the roof rack could be a parasitic radiator that's causing a reflection. SWR is a measure of the ratio of power sent forward out of the radio against power reflected back. But it's nearly impossible to tell if the reflection is due to a bad connector, kinked coax, bad coax-to-antenna connection or power reflecting from nearby objects. So you have to work through and eliminate them. Since your reference antenna has a low SWR, you can probably safely assume the radio through mount connections are OK, so it's an interaction of the antennas themselves.
Could also just mean the antenna needs to be tuned. Do there happen to be a couple of Allen screws holding the whip on? If so, try loosening them and making the antenna longer or shorter and see if the SWR changes.
"Silent enim leges inter arma." -- Cicero