wanrep

You might want to pull the drive clutch off and make sure everything is sliding freely. Inspect the rollers for flat spots.

When it won't pull, is the engine bogging down or not. Trying to determine if the clutch is slipping.

If you're not hearing any sound like it's going into gear, you'll have to pull the left engine cover off and inspect the shift shaft and and gear cam. The gear cam is pinned to the transmission shaft. Remove the bolt holding it and make sure the pin isn't sheared.

Check that the gearshift lever, on the left side of the motor is moving normally when you operate the shift handle. If it is, take the belt off, put it in gear and try to turn the driven clutch by hand. If it turns easily, it's either not going in gear or the input/driven clutch shaft is broken. I've never seen one of these shafts break but that doesn't mean it can't happen. Neither drive shaft is moving when you rev it in gear, correct? Does it sound like it's going in gear when you shift it into gear?

Could be just a worn belt.

Yes, I'd start with the belt.

First place to look is at the shift mechanism located behind the clutch, behind the right engine cover. You can pull it out and see if it's working properly. If it is then it's likely in the transmission itself which requires splitting the cases.

When the brushes in the starter motor get worn down, the starter will work intermittently. Try tapping on the starter motor with a hammer while pressing the start button next time it doesn't start. If it suddenly turns over, it's the brushes in the starter motor that need replacing. You can buy a starter brush plate for $25 instead of buying a whole new starter.

Regarding the engine number....4WU is the model code for YFM350FWJ, Big Bear SE 4WD. 1997-1999

Find some used rear rims from a 650/700/750 Brute Force or Prairie. They use a 25x10-12 tire. You'll have an endless choice of tires and they'll be a lot cheaper than 25x11-10.