Taxable use and addition of lift axle

We purchased a new truck for use in our business. The vehicle was not subject to Section 4051 tax (FET) because its GVWR was at or below 33,000 pounds. We want to add a lift axle to the truck that will increase its GVWR to above 33,000 pounds (and we will do the work ourselves). We do not plan to resell the vehicle, but simply use it in our business. Can our mere use of the truck trigger FET? Also, the cost to add the lift axle is well below the 75 percent threshold for further manufacturing. Will that affect whether FET applies?