essentially a spool eliminates the diff and makes the rear end, in lamens terms, a solid axle.
That means no matter what, both wheels will scrub in a corner because they both rotate at the same speed, adversely affecting how the truck handles.
When using thick/heavy fluids in a diff on an R/C vehicle, it essentially becomes a Limited Slip Diff (LSD), but remains a diff in that it can disperse power to either wheel by allowing each wheel to rotate either faster or slower in a corner/ when the vehicle is turning.
Centre diffs are a different matter.