Highlander Shake

I have a 2002 Highlander V6 AWD taht shakes hard at 45-55 mph. The dealer has looked at the car, put new tires on, alignment, and still cannot locate the source of the shake. I seems to be coming from the front end but does not shake the steering wheel. The dealer wants to swap components from another 2002 highlander but do not have any on the lot. Any better suggestions to isolate this problem.

What components does the dealer want to swap?

The back differential. But I think it’s in the front end.