While in 4lo driving up and down the sandbar at the river the other day my transmission didn't want to shift out of first. I shifted the transfer case thru the gears and the transmission, it started shifting fine. I have had no problems before this. The Jeep acted up 2 more times after that as well. No problems shifting lately and fluid is nice and red and on the full mark. The transmission had recently been serviced, fluid and filter change. The transfer case is a 231, recently replaced due to a drive shaft incident. Does anyone have any thoughts what could've caused the.shifting problem?