It could be a number of things. If it's drivable I would take it to AutoZone and they can do a check and tell you what the problem is. It's also free so it wouldn't hurt.
Check my isuzuparts.com
Check this Isuzu site: http://www.myisuzuparts.com/
How do I reset the CPU in a 1999 Izuzu trooper. After the battery went completely dead at will not start due to the cpu. (Battery now charged
Flush radiator
Check myisuzuparts.com
I have a 1994 Isuzu Trooper and it has issues upshifting at lower speeds. How do I check the fluid level in the transmission when there is no dipstick?
If the check engine light is on...might be a dirty EGR valve.
check your tranny fluid first
How to check trans fluid on a 1997 Isuzu Trooper
if it is an automatic then yes, if it is a standard transmission then no
Someone can find an Isuzu Trooper for sale by checking their local Yellow Pages and locating an Isuzu dealership near them. Also, you can check your local newspaper's classified section for any listing of new or used Isuzu Troopers for sale.
check the engine temperature sensor. the trans won't go into overdrive until the engine is warm.