I spent $500 (parts only, I do my own work) replacing axles, wheel bearings, ball joints and completely disassembling and reassembling the brakes on my TDI Jetta chasing a vibe. It turned out to be an out-of-balance wheel. I had the wheels rebalanced initially but it wasn't done properly and the symptom remained.

Start with the cheap stuff and work your way up.

FWIW, I put CVJ axles on my 300D and it cleared up a bunch of vibes both driving and idling in gear. One was so shot that it was binding up. How are your axles?

__________________
-----------------
1984 300D 193.5k miles
ASTM-certified B100 bio when able