Any advice on a '95 Grand Cherokee ZJ that won't go into overdrive?

Hi! My jeep has about 165,000 miles on it and won't switch into overdrive-although the overdrive switch light is always on.
The overdrive kicks in occasionally, but for the most part it won't get out of third gear, I'll be driving 65 mph at 3200 rpm and
feel like if I go any faster the engine will completely overwork itself. Thinking first it was a mechanical problem, I replaced
both the ignition switch and the right pod switch (which controls the O/D, rear wiper, and rear defrost) but with no change.
We are thinking next of trying something with the solenoids in the transmission . . . has anyone else had this or a similar
problem, and if so are there any suggestions you can give me in trying to track it down? I appreciate any advice, thanks so
much, take care!