I have a 1996 Toyota 4Runner.
When I drive my car at normal speed of 45 mph all of a sudden the RPM revs up high and then I cannot accelerate past 20 mph. What could be the problem? Someone told me I had to replace the transmission is this true?
Replacing the transmission is not required. You may have a leaking input shaft seal on the transmission that is contaminating the clutch plate with gear oil, causing slippage under load. Further to this, the engine rear main seal may also be contaminating the clutch with oil.
Check the engine and gearbox oil levels initially, to help diagnose. If these checks are done correctly, and fall within tolerance,then unfortunately the clutch drive plate friction material or pressure plate tension may have worn out with age and needs to be retired.