I would probably create a single table for Units and give a unique ID to each truck and equipment. The ID would be a SKU built from a pre-defined set of rules describing how it's formed. For example, every truck ID would be prefixed with a T, and equipment could all begin with E. This would simplify your orderDetails table because then you could reference a single ID regardless of the type of Unit being leased. I would likely use a separate table then to store Unit attributes.
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