93' f-150 shudders in OD/cruising speed

hi, I have a 93' Ford F-150, with a 351 engine, electronic 4 wheel drive, automatic transmission, XL, single cab...anyway, at about 135,000 miles it developed a minor "shudder". It only shuddered when I shifted into overdrive and when cruising at about 55-65 mph. The shop said it was my transmission, so I just left it and figured I replace the transmission whenever it croaked. Well, it ran another 20,000 miles, with the shudder getting worse and worse, sometimes doing it while in 4th gear, but usually just in overdrive. At about 155,000 miles, the transmission finally died, so I replaced it with a used one from a wrecking yard. Totally different transmission and torque converter, and the shudder is still there, as bad as ever. It's not a u-joint or rearend because it is directly linked to the shifting process. Would it be the transfer case? I thought too it could be something in th engine, like it shudders when the engine is under a certain load point, like when it reaches overdrive? I'm stumped, Its been in and out of the shop, i've looked for that shudder off and on for 2 years. Any ideas?

my company had a 94 that did that and we always assumed it came from the rear differential.
totald truck before we ever figured it out

Make sure it is not a engine miss.
check the tune up end of it.
Mt

When the shudder starts, if you let off the gas, does it keep doing it? If you give it more throttle, but not enough to shift out of od , is it worse? If you put it in neutral does it go away? Could be like MT said, could be a miss under load! I would check the driveshaft though to see if its bent , or maybe a joint is actually bad. Bad joints can cause some pretty curios vibrations sometimes, that can be speed/load related. Good luck.
Later
Ed
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Stop by the parts store and buy either Lube Gaurd or Smart Blend and pour it in your trans fluid. This is usually a torque converter clutch chatter. If you fluid is cooked this wont rebuild your trans but if you fluid is red this wil buy you some time.

Forgot tooo add if you let this go to far it ruins the front pump of the trans and it will need an overhaul. This is very common these transmissions

Forgot tooo add if you let this go to far it ruins the front pump of the trans and it will need an overhaul. This is very common these transmissions
Yes do the LUBE GUARD it worked for me,, also change fluid too,
its still red and smells like good fluid...