Buckeye, depending on your transfercase you may or may not be having a transmission problem. If it's a part time 4wd it's the transmission, if its full time, with quadra track it might be the transfer case. The full time transfer cases use a fluidic clutch system similar to a torque converter in operation, if it loses too much fluid it will slip just as you describe, but so will an automatic transmission. The transfer case is more forgiving, but it only has about 3 pints of fluid in it,,, not very much. However loss of enough fluid in a transmission to cause this amount of slipping is a very serious issue and can cause other damage to the transmission. Check fluid levels in both if the tranny fluid is up to the proper level, not burnt smelling or foamy then it probably is the transfercase providing it is a full time 4wd system. if its part time, look to the transmission.