my truck wont shift into the overdrive or 4th gear for highway travl we have checked the vacume lines and the wiring any suggestions on what to check or how to test? also was running fine then wont run except on idle and is rough. and a question does anyone have the drivers side seat belt or know where i can order or buy one?
The transmission won’t shift into overdrive (lock up the torque converter) until the engine reaches operating temperature.
Check if there’s a problem with coolant temp sensor for the computer.
Tester
i drove it for 2 hours and it still wouldnt shift into 4th gear
Well?
If the coolant temp sensor for the computer isn’t working right, you can drive it to Timbuktu and back, and it won’t shift into 4th gear.
Tester
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1988 was the first year for an automatic transmission with an overdrive gear in a Dodge Ram 150, 1984 Ram has a 3 speed automatic.
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@Nevada_545 is correct. It won’t shift into 4th gear, because there is no 4th gear. That truck has the vaunted A727 Torqueflite. Bulletproof transmission, but no overdrive.
then can you explane why when my dad drove it he was getting 18mpg and im getting 11
how do i lock up the torque converter?
Faulty coolant temp sensor for the computer.
the computer thinks the engine is always cold.
Tester
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You have to go through the basics to deal with the rough idle. Plugs, air filter, plug wires etc. Lots of other things to look at. What maintenance history does the Ram have? Driving habits can have a big influence on mileage. Your dad could have had a gentle foot on the gas, you may be accelerating hard. But until you should get the rough idle figured out first.
Several possible answers:
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Your dad was wrong; that truck is rated at 17mpg highway, 14 city. Getting 18mpg out of a 17mpg truck that’s more than 30 years old is either an inaccurate mpg calculation, an amazing fluke, or your dad is one of those hypermiling goofs who puts everyone else in danger driving slow and running stop signs to save on gas.
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something is wrong with the engine now that wasn’t wrong with it when your dad drove it - see @Tester’s reply
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You have a lead foot and your dad didn’t.
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Sorry, that’s just not true in general. My forester is EPA rated at 32 MPH highway, I get that easily at 70 MPH. If I drive at 50, I get 38-39 MPG. And I drive those speeds under cruise control, so at a constant speed.
Assuming your car is the first model year ever made of the Forester, it’s 13 years newer than OP’s truck and has much more advanced fuel-saving technology on board.
OP drives a 34 year old truck with, depending on the engine, a powerplant that may well have been designed in the 1960’s. He’s not gonna beat EPA mileage ratings on that thing unless he drives it really gently and even then he’ll probably need a good tail wind to do it.
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i do not have a lead foot nor does my dad but ok
Great. That eliminates that one. It’s probably one of the other two possible causes I suggested. 
So you are willing to drive 10 hours on a 7 hour and 10 minute, 500 mile, interstate trip to save 20% in gas. I am not.That is about $8.10 for driving an extra 2 hours and 50 minutes. About $3 and hour, I won’t work for that.
If the truck has Lean Burn with electronic spark control you might save a lot of grief retrofitting mid 1970s carburetor,distributor and ignition module while you’re doing a tune up.
ok so i have the truck on the road and driving it consistantly but there is still a major power lack in the engine does anyone have any idea of what i should or can put in to improve the power but not lose a lot of mpg?
Yes, trade it for something a lot newer . It was never a power house or miles per gallon champion in the first place.
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Your truck was built during the dark days of smog control. It is before aluminum heads and fuel injection. To meet smog standards, it had low compression, retarded timing and a lean running carburetor.
I think the first thing I would do is check the advances on the distributor. You have both a vacuum advance and a centripetal mechanical advance. By now, the vacuum advance diaphragm has probably rotted away. These are pretty cheap at any parts store, but you may have to order it.
The weights and springs on the mechanical advance may have gotten stuck also, Just pop the cap off and look for the weights. They maybe located under the reluctor plate or just under the rotor. A little spray oil on the weights and springs will usually free them up.
When you have checked/repaired these, then get a timing light and check the timing both for initial timing at idle and timing advance at higher (~2000) RPM. If/when that is good, then check all your vacuum lines and possibly look into a carb rebuild or a new carburetor. There maybe an aftermarket carb that will work on your vehicle that will provide better power and fuel economy.