First off, the wheel should be 17.5" diameter, not 17"--have you called DRV parts for Greg Vaughn?
This is the cover that goes over the axle hub? Is it the one that fits from the inside of the wheel? Best way to find one is to take one of the remaining three to a wheel accessory shop--have seen several with center caps laying around on display. Use the old one to find the right size.
Also, identify the mfg of the wheel might help locate correct cap.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died