Funny shifting ake at 50 mph like shiver in transmision

probably a faulty transmission control module BUT run the fault codes to find out exactly

First thing I would check is the transmission fluid level, and it is not going to shift at 50 MPH if you are in 4WD. If it has been driven on the clear highway in 4WD at high speed then most likely the rear end is going out. From there it would be most helpful to know more details how the vehicle was running shortly before the shifting problem started. Engine size and any details you think might help will be most kind, as I can't give a proper solution or suggestions with the info listed. Feel free to email me with details and then I can help with this. Hope this Helps.

its probably the kickdown cable look under the hood its the cable that runs next to the throttle cable see if there's alot of slack or if its too tight and not letting it release all the way.

transmission clutches bad for third gear operation. needs trans overhaul..

sounds like over drive switch or transmision box

check the atf fluid level first if thats full up it could be a blocked filter within the gear box worst case is the box its self it could be slipping on you,

At 45 or 50 is the engine RPM normal? If its higher that it should be transmission could be slipping.
If the engine is misfiring at 45 or 50 it could be the spark plugs.
If its just holding back on power it could be catylitic convertor, or possibly fuel filter.

take it to cottman transmission

if there is fluid in it take a sniff of it if it smells realy burnt you may want to take it to a trans shop isuzus especialy rodeos and troopers are known for bad transmisions sory