99 dodge 4x4 auto shifting problems

This truck is a 1/2 ton w/ 360cid, w/towing package, has about 90,000 miles on it now. This started at about 65,000 miles and is getting slowly more frequent.
The first time out of the gate it seems to shift normally. After that it needs to get to 3000 rpm to shift from 1st to 2nd and from 2nd to 3rd. 3rd to overdrive works fine. This doesn't seem to be affected by cold or warmer weather. If it were older I would suspect a vacume modulator. This is my dads truck, so i havn't been able to look at it yet, any help would be appreciated.
Kevin
You may want to start by hooking a backpressure guage into one of the oxygen sensor bungs. If you have a restricted exhaust, the engine won't rev up also causing trans. shift problems.
I also have found 2 technical service bulletins for this truck if you have the 42RE transmission. One is for band adjustment #21-16-98
One is for PCM Reflash #18-030-01
If you google technical service bulletins and then type the numbers you should be able to read them in detail.
Good luck and let us know how you make out.