delayed shifting into reverse

On my 98 Grand Cherokee Laredo with the Selec-Trac, the transmission seems to take about 2 seconds to shift into reverse from Park or Drive. When I shift it into drive from park or reverse, it shifts immediately and when driving, the transmission shifts smoothly and quickly. If I shift quickly from reverse to drive and back to reverse, it then shifts quickly into reverse. But here is the thing; sometimes when I shift it into reverse and give the Jeep too much gas, it seems like the transmission slips and then jumped back into gear only when I start moving backwards. After that, I can hit the gas as hard as I want going backwards and it doesn't slip or anything. Now I checked the fluid for any leaks for any bubbles or burnt fluid and it is full and the fluid is pretty clean (which leads me to believe that the transmission has been maintained before I bought the Jeep). It does have 144,000 miles on it, though.

Can anyone help me out here? I want to fix this problem before it gets any worse. Thanks.

and to add one more thing...could it possible be just the reverse band in the tranny needs tightening?