88 chevy Silverado 4x4 2500

I had a similar problem with my GMC,,,same family as Chevy. Tech said the reason for the hard shift, was the computer knew some type of transmission slippage was happening, so it automatically increases the shift pressure to insure the clutch discs do not slip and burn. (Do not know if this is true or not, but when transmission would not shift into OD, all shifts were very hard/jerky shifts like it had a shift kit installed). He said some type piston in torque converter was busted/cracked and would not allow the torque converter to maintain the proper oil pressure to stay in the lockup position. All I know, is a new torque converter fixed the problem, just a mechanical failure. Tech said GM changed the torque converters in the 2000 models, had a number of failures and changed back to the old setup. Again, do not know if this is true, but a $1000 torqure converter replacement fixed the problem Maybe this will narrow your focus

The problem you are explaining also can be a solenoid problem. You can buy the solenoids either on ebay or chevrolet. They are not hard to do.

Here is a 99-06 chevy video on how to adjust the shift cable......this may be of interest https://www.youtube.com/watch?v=aek4FylgWR4

Are there any lights on ur instrument cluster? A check engine light? Did u get it scanned yet for codes?? If no codes, its ok