He guys

My Fbody, '91 with an 4sp. aut. Has a weird issue. When driving the freeway, when the car shifts to 4th gear, the car starts to shake. I had this issue last week and tightend my frontwheelsbolts and the problem was gone. Today, cruzing for 10 minutes on the highway, the car started to shake again.

I checked the driveshaft during an annual checkup and me and the mecanic concluded that's There's no problem

I think my frontwheels need to be balanced.

What could it be more, as it only happens in 4th gear...ergo: driving in highspeed (100km)

If more info needed, let me know!