To start, try to fix the 99 Ranger first.
Being a manual transmission, there should be no felt slipping (if there is, thats not good).
Start checking these first:
Proper clutch operation, try stalling the engine by gently releasing the clutch without giving any gas. If the engine stalls, the clutch is locking up (good thing). Also check inside the bellhousing for any fluid leaking. The slave cylinder is know to go out and cause havoc (clutch not releasing or slipping when soaked with fluid).
Check to make sure the vacuum hubs are disengaging. It could be that you are feeling the extra rotational mass from the front wheels. Lift the front tires in the air, engine running, 2wd mode, Neutral gear, manually spin the tires. The CV joint should not move along with the tire and/or the opposite tire should not spin. Also check the 4wd system. Engage the 4wd with the front tire in the air, engine running, tranny in Neutral and spin the front tire manually. Opposite side tire should spin.
Check the parking brake cable. Make sure its loose enough not to drag the rear brakes. But not so loose that it doesn't work either. Pedal should go about half way down before engaging, but not to the floor.