My 1998 4.0 runs about 40 lb. with rpm's, but drops to the 20-25 lb. range at idle. Thats with less than 28,000 miles on the engine. I bought the Jeep with 4,000 miles on it and it hasn't changed since then, so yours sounds to be o.k. to me.
My 99 4.0 was around 42 at cold start 24 after warm up off idle or going down the road ..and 12-14 hot idle. I swapped in a high volume pump. It appears from some unofficial surveys ..that the "standard" oil pressure in the 4.0 varied quite a bit ..even within the model year.
The fluctuation is a bit odd ..you should be at your MAX pressure COLD at any decent road speed. Once warm ..it should be somewhat lower ..but just as stable and predictable. Your sensor may be faulty.