I had the same thing happen in a 4x4 offroad. I logged out and back in I still could not remove it. I restarted the server that did not help. I have tried double clicking the barrel, tried dragging the barrel nothing works.

I just found another offroad in Zub. I got int the drivers seat, seat in it for a min or so. Logged out, restarted the server and logged back in. The offroad was still there. Not sure if I would trust leaving gear in it, and trying to logout in it everytime or not.

I got in the drivers seat of a V3S, and tabbed out and restarted my server. I logged back in and the V3S was gone. Maybe I should have waited a few mins to restart the server. I'll try that next time.