I do not know why Lionel has so much slop in its wheel sets on diesel locomotives. This excessive slop is ridiculous nonsense and not needed; especially on a 4-axle unit.
I have seen this before on my purchased units. Here is what I do. I take one or two 671M-22 e-clips and clip them onto the axle opposite of the gears to make sure that the gears remain fully engaged. The Santa Fe GP30 below has been running like this for at least 6 years. If you look at the wheels opposite of the gears there are two 671M-22 e-clips on the axles.
Gears that do not mesh properly wear excessively and will fail prematurely.
All my units have at least one e-clip on the axle.