Is this causing my low speed vibration issue?

[64stpside]

Hi guys. Well......I took the 64 out sunday afternoon to check the brakes after i installed a new master cyl and brake lines. Stops great. My issue is when I got to about 5-10 mph, the front end started shaking. Here's what Ive done to the front end so far (64 step side,2wd). New shocks (rears too), pitman arm, rebuilt drum brakes. Tonight i'm gonna jack the control arms up and check the ball joints. What I think might be the culprit is that it has aftermarket wheels on it (10 in wide). There are no centering rings, nothing but the lug nuts to line up the wheel. I installed some aftermarket center caps off ebay (all were missing when I got the truck). The lip of the center cap is not seated in the back of the rim as there is no provision in the wheel for it to sit flush. It's not sticking out bad but I wonder if the wheel is not sitting flush against the drum, is causing the issue? Oh, I have a 1 piece drive shaft so no carrier bearing to check.
Before anyone asks if I felt a difference after I put the center caps on, the truck came home on a trailer and it's been a yr getting it going so this was my 1st real test drive.
Do you guys think hub centering rings will fix my issue or does the fact that the center caps not being flush with the back of the wheel, will still cause an issue.
Here's a link to the wheel I think are on it. They look exactly the same. It's the chrome modulars
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=70