Hi Jordan
I agree, it's not a decoder issue and as Steph has mentioned I use a Loksound L decoder to drive a Pittman motor and two speakers in a Red Caboose GP9.
I'll hook up my spare Loksound L to a Weaver drive loco over the weekend to see what happens and report my findings.
I would have checked what you have already checked and a few more:
Have you tried operating the loco on an isolated section of track not connected to the layout?
Are you able to disconnect the lead truck and try the decoder with the rear drive truck to see is that works satisfactorily?
Other things I would also check for if you have the loco apart are split gears in the gearboxes as I've had weaver gears split and the drive shaft connection, again I have one split and is held here with a cable tie.