1988 1500 700r4

I noticed when the truck is shifting from 3rd to fourth I get a vibration ,like I was driving over a washboard. This has been going on for 6 months. The other day I didn't think I would make it up my driveway, which is a little steep. I know nothing about tranny's. Any ideas? oh , btw, It's got 170,000 miles on it. It's been a great truck over the last ten years that I've owned it. LWB ext. cab. 18 mpg. Great 350 engine.

I should also say that the vibration only happens occasionally and lasts for a couple of seconds.

Sounds like the clutches are slipping, change the fluid lately ?
It may be on its way out.