1998 dodge ram 1500 A/C only blows through defrost

The air flow controls are vacuum operated and if the vacuum is lost it shifts to the default or no-vacuum position which is to the defrost. You most likely have a broken plastic 1/8" line under the hood near the RH side of the engine that supplies the vacuum to the HVAC controls. If the controls work normally unless you are accelerating then switch by themselves to defrost you may only have a bad check valve. There may be a vacuum diagram posted on the underside of the hood that shows these lines otherwise you'll just have to examine each line until you find the bad spot. There may be a plastic vacuum reservoir that one of these lines goes to so check it for any cracks or damage also.

I read up on line some reasons and found a broken vacuum line two inches from the fire wall. The previous day I had changed spark plugs and must have disturbed a brittle line while struggling with the rear plug. I used a small diameter shrink wrap electrical tube to mend it. all working well including my cruise control that had been acting up.