I have been digging out the original LL Proto 2000 HO scale GP20s to put new gear sets on the wheels,One worked fine afterward but as for the other unit the locomotive still barely moves. I have taken it apart, checked the worm gears on the drive, cleaned out the old grease and regreased the gears. After putting it back together, it still barely moves..the drive shaft moves for a second, then stops. It does that in either direction (the directional lights still work).
Could the motor be bad? the brushes?
It sounds to me like the motor may be in need of servicing. I'd try cleaning and oiling the motor first. (Unless you've already done this, that is). Another thought would be that there's a bind somewhere in the drivetrain. Have you tried turning the flywheel/driveshaft by hand to see if it feels stiff? Yet another thought is that there's an electrical gremlin somewhere that only affects the motor. Is this loco analog DC, or DCC? If it's analog DC, you should be able to hookup test leads directly to the motor's power pickups. If it runs this way, you have an electrical issue. If it still won't run, then the motor is probably bad.
Take a few minutes to carefully remove both the worm gear retainer clips. As you know these also hold the trucks in place.
Proto_gear-7 by Edmund, on Flickr
With each worm gear removed you can roll each truck and determine if one or both of them are turning freely.
Proto_gear-4 by Edmund, on Flickr
Sometimes the axle doesn't quite sit properly in the metal bearing retainers. Check these. Also with the worms removed you can check the motor rotation. I have some L-L engines that are 25 years old and probably have a thousand hours of running and I don't recall ever wearing out a motor brush. I have cleaned the commutators on some and reduced the spring tension a little on others but, generally the Life-Like motors were pretty decent.
Good Luck, Ed