If this set is from around 2005-2006 the truck probably is a victim of zinc pest. MTH is aware of the problem. If you call Midge, explain the problem along with the set number they will issue you an RMA. Send them all your broken trucks and they will replace them no charge other than maybe shipping. I exchanged mine at York so shipping wasn't an issue.
You can epoxy them but they will just break somewhere else.
Pete