1990 Ford F150 4.9L L6 Overdrive transmission issue.
 

I have a 1990 Ford F150 with a 4.9L L6 (automatic)... The issue I am having is that when I put it in drive it acts as neutral. When I shift to drive, I feel a tug as if it connected but when I give it gas, rpms go up but the truck won't move. I'll switch the Overdrive off (button located on right next to steering wheel) and everything works fine and from 45 mph to 55 mph, usually when I get on the highway, I switch Overdrive on and as long as I don't give it too much gas it'll catch. But.. If I give it too much gas the transmission seems to slip and engine just revs...

Back to the past... It started one day when I had multiple buckets of stain in my bed and more weight. I was keeping a steady 70 mph speed on the highway and the truck tugged back and transmission slipped. It connected again but then slipped again. So I got off the highway and drove in 1st and 2nd. Then I discovered that switching Overdrive off I could drive it no problem.

If anyone could please help me maybe understand the issue I'm having so I can repair it I would appreciate it very much.

***Also note that Park,reverse, 1st and 2nd all work fine and that I've changed the fluid, the filter was NOT dropped in the pan, I changed all spark plugs, I haven't looked at any cables or anything further than that.***

Don';t know without checking, but suspect internal problem with OD servo piston losing trans pressure allowing clutch slippage. If driven enough with clutch slipping trans Ovhl or Replacement up the road. When you had the pan down should have looked for broken or visual problem with OD area. May need to remove valve body to do this.