94 ford bronco with a e40d transmission wont go into 4th gear. Any idea on the problem?

The transmission has been staying in 3rd gear and it wont shift into 4th. It also tugs back and forth when it reaches 3500 rpm. It wants to shift into 4th but immediatly goes back into 3rd. Already replaced the vss and mlps. What else should i look into? Thanks in advance.

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