6G rattle/vibe at 1650 rpm

This is my first post - I'll post up an intro with some pics soon, but for now, some particulars...

I recently got a 91 Eagle 2000GTX (basically the Canadian version of the seldom seen Galant GSX, which was an n/a 2.0 AWD, no 4ws). Oh mine's a manual transmission.

It had recently had the timing belt done (PO had receipts from the shop he used), it was sitting for a while, and needed a battery and some bodywork, so I bought it for a good price, and trailered it home... There was a fair amount of slack across the cam gears when we pulled the cover to see how the belt looked. We ended up going in and retensioning the t-belt, and then I replaced the acc belts just in case. Also did all coolant hoses, t-stat, plus plugs, oil change... standard tuneup stuff. Oh, and a nice Optima Red Top.

But now finally driving it, I am getting a substantial rattle/vibration at 1600-1700 rpm. This happens in neutral or in gear, under load or not, seems totally related to rpm, with other factors seemingly having no effect. We checked the engine mounts, and fixed an exhaust hanger but the vibe seems to be more central to the engine than anywhere else. It's the kind of vibe that builds as you reach that magic number, but really rattles at ~1650rpm, and then rapidly drops off as you continue to climb the revs...

No loose heat shields. Nothing apparent in the driveline. Not speed dependent. Only RPM dependent.

Is it possible that the shop that did the timing belt got the balance shaft out of phase? I've done searches on several forums, and although this seems to be a common problem, the posters that have had this issue seemed to have the vibe much higher in the revs, and across a range of rpms, as opposed to a specific spot.

Does it sound like a balance shaft issue to you guys? I know we can strip it down again and check, but rather than doing it all for nothing, any opinions first?

Cheers,
Phil